[Why?] The reason why mortgage interest rates follow along when you're happy about deposit interest rates rising lol
“Deposit interest rates have gone up!”
For someone who has lived in a world where I’ve long wondered, “What is interest, anyway?”, this is actually quite pleasing.
The money I’m just keeping in the bank earns more interest than before.
The bank has finally noticed my deposits, meow.
……However, if you look at someone who has a mortgage, they can’t really laugh about it.
Mortgage interest rates are rising too.
“Wait a minute. With deposits, I’m the one receiving interest. Why does the side I’m borrowing from move in tandem?”
Let’s clarify one thing first.
It’s not that mortgage interest rates rise because deposit interest rates rose.
Both are influenced by changes in a larger “interest rate environment,” such as the policy interest rates and market interest rates behind them.
And interestingly, they don’t all move at the same time.
In August 2026, the Bank of Japan explained that regarding the movements following recent rate hikes, the increases in ordinary deposit interest rates and short-term prime rates generally center around two months later, while new lending rates for variable-rate mortgages are slower, and application to existing loans is usually even later.Bank of Japan Website
Deposit interest rate: “I’ve gone up!”
Me: “Yay, meow!”
Mortgage interest rate: “I’ve arrived late.”
Me: “You’re coming too, meow?”
This time, let’s track why this “interest received” and “interest paid” start moving in the same phase.
📜 Chapter 1: It’s not like deposit interest rates alone get better on their own
First, let’s think about banks in a very simple way.
There are people who deposit money in banks.
On the other hand, there are also people who borrow money, such as for mortgages or corporate loans.
From the perspective of a depositor, interest is something you receive.
From the perspective of a borrower, interest is something you pay.
Even though it is the same two characters for “interest rate,” the meaning changes completely depending on your position.
It is as different as the story of giving a cat a treat versus taking a treat away from a cat.
So, why do both move?
One of the factors behind this is the Bank of Japan’s policy interest rate.
When the Bank of Japan influences short-term interest rates through monetary policy, the impact spreads to financial institutions’ deposit and lending rates via market interest rates.
Therefore, the flow is:
The policy interest rate moves.
Short-term market interest rates move.
Changes spread to bank deposit and lending rates as well.
That is the sequence.
Of course, not all banks change their rates by the same amount on the same day.
If you understand this as “because the Bank of Japan raised it by 0.25%, both deposits and mortgages will all go up by 0.25%,” the logic falls apart.
In fact, the Bank of Japan itself explains that there is a time lag between a rate hike and its ripple effect on various interest rates. Bank of Japan website
In other words, deposit rates and mortgage rates are not so much like twins holding hands, but rather
two people walking at different speeds in the same weather
is a closer way to think about it.
If it rains, both get wet.
However, one is already running to grab an umbrella, while the other is still at the entrance putting on their shoes.
🧩 In other words, what does this mean?
“Deposit interest rates rise”
↓
“Therefore, mortgage interest rates rise”
That is not the case.
“The interest rate environment changes” → “Impacts deposits” + “Impacts mortgages”
This is what it is.
If you only look at deposits, “interest has increased.”
If you only look at mortgages, “the burden might increase.”
Only those who have both take the dual nature of interest rates head-on.
Monetary policy does not have a one-way service spirit.
📜 Chapter 2: Moreover, mortgages come chasing after a little late
This is where things get strange this time.
Bank of Japan Deputy Governor Ryozo Himino explained quite specifically in August 2026 about the time it takes for interest rate hikes to be transmitted to deposit and lending rates.
According to recent performance, many financial institutions primarily raise ordinary deposit interest rates and short-term prime rates two months after a rate hike.
However, new lending rates for variable-rate mortgages are slower than that.
It is said that it is normal for the new applicable interest rate to be reflected in existing variable-rate mortgages even later.Bank of Japan Website
This is what makes human perception complicated.
Interest rate hike.
After a while, deposit interest rates rise.
Oh, the interest is going up.
More time passes.
The mortgage side has arrived too.
Joy and burden do not arrive on the same day.
It is a time-lagged delivery.
Moreover, it doesn’t end there.
With existing variable-rate mortgages, just because the applicable interest rate has risen does not necessarily mean that the monthly repayment amount changes at that exact moment.
The Bank of Japan explains that because many mortgages have a ‘5-year rule’ or similar, the monthly repayment amount may not change until even later after the applicable interest rate changes.Bank of Japan website
This part is quite important.
Rising interest rates and rising monthly repayment amounts are not synonymous.
Note that the 5-year rule does not exist for all mortgage products.
The handling differs depending on the financial institution and product you have contracted with.
Therefore, when checking your own mortgage, you need to look not just at the news that ‘variable interest rates have risen,’ but also at what criteria determine the interest rate in your contract, when it is applied, and how the repayment amount is reviewed.
Even if you immediately start crunching numbers after seeing interest rate news, your contract is waiting, saying, ‘No, read me first.’
And in fact, the short-term prime rate, known as one of the benchmarks for bank lending rates, is also moving.
According to Bank of Japan statistics, the most frequent value for the short-term prime rate of major banks rose to 1.875% in March 2025, 2.125% in February 2026, and 2.375% in August 2026.Bank of Japan website
Lined up as numbers alone, it seems modest.
However, in a world where you are repaying tens of millions of yen in loans over a long period, like a mortgage, ‘the interest rate number moved a little’ does not necessarily mean ‘the impact is also a little.’
That is precisely why, before you raise a glass in celebration just by looking at the interest in your passbook, it is easier to understand the whole picture if you also take a look at the borrowing side.
If it were me, I’d raise a toast.
I’ll check after that, meow.
It’s the order in which we get scolded by the boss.
📜 Chapter 3: There’s another trap when you lump it all together as “mortgage interest rates”
What makes the story even more complicated is that
there are also types of mortgage interest rates
that exist.
The most representative ones are the variable interest rate type and the fixed interest rate type.
As the name suggests, the applicable interest rate for the variable type can change during the loan period.
The relationship with short-term market interest rates and the base interest rates set by financial institutions becomes important.
On the other hand, for the full-term fixed interest rate type, there are products where the interest rate after borrowing is fixed until the end.
For example, “Flat 35,” provided by the Japan Housing Finance Agency and private financial institutions, is a full-term fixed interest rate mortgage where the borrowing interest rate and repayment amount are determined at the time the funds are received until the end of repayment.Flat 35
Furthermore, the background to that is also interesting.
In a standard Flat 35, a mechanism is used where the Japan Housing Finance Agency purchases mortgage loan receivables from financial institutions, issues bonds backed by them, and raises long-term funds.Flat 35
In other words, even for the same “money borrowed to buy a house,” the mechanisms that determine the interest rates are not all the same.
While a person with a variable rate is watching the news, a person who has already borrowed at a full-term fixed rate might say,
“My contracted interest rate is fixed, though?”
This can happen.
For Flat 35 as well, although the most frequent interest rate for new loans with a repayment period of 21 to 35 years is 3.46% per annum as of September 2026, the borrowing interest rate for someone who has already contracted at a full-term fixed rate will not be automatically replaced by this new interest rate later.Flat 35
Lumping all of this together,
“The Bank of Japan has raised rates! Every mortgage holder will see their rates go up by the same amount starting tomorrow!”
If you think that, you are spectacularly wrong.
In the world of interest rates, the more similar the names are, the more dangerous it is not to look at them individually.
🔎 This is a point that is often misunderstood
One more thing.
The “5-year rule” does not mean “interest rates will not rise for 5 years.”
Even in the Bank of Japan’s explanations, for existing variable-rate mortgages, the “time when the applicable interest rate changes” and the “time when the monthly repayment amount changes” are explained separately.Bank of Japan Website
The part that might not change for 5 years is the repayment amount, not that the applicable interest rate itself is fixed for 5 years.
If it were a variable rate but fixed for 5 years, the name alone would start a family meeting.
And there are products that don’t even adopt the 5-year rule itself.
So in the end, what wins for your own loan is not general theory, but your own contract terms.
Now, having heard this much, let’s ask our usual trio about interest rates too.
🍶 The interest rate board is buzzing again tonight
🍶 Former bank employee, waiting for the return of time deposits: When deposit interest rates rose, for the first time in a while, it felt like “there is interest.” It’s not the same view from the borrower’s side, you know.
🏮 Retired person who bought a house with a variable rate: That’s the thing. It doesn’t end with “deposit rates went up, I gained.” I’ve gotten into the habit of looking at mortgage news as a set.
🍵 Former section chief who was on the fixed-rate side: But if you just say “mortgage rates went up because deposit rates went up,” the order is wrong. Both are receiving changes in the interest rate environment.
🍶 Former bank employee, waiting for the return of time deposits: Exactly. It’s like getting separate messages from the same parent.
🏮 Retired person who bought a house with a variable rate: And the arrival dates of the messages are different.
🍵 Former section chief who was on the fixed-rate side: According to the Bank of Japan’s explanation, there is a time lag in order: ordinary deposits, the short-term prime rate, new variable mortgages, and application to existing loans.Bank of Japan Website
🏮 Retired person who bought a house with a variable rate: Just send them all at once. I’d only have to prepare myself once.
🍶 Former bank employee, waiting for the return of time deposits: Monetary policy isn’t going to adjust to your emotional readiness, is it?
🍵 Former section chief who was a fixed-rate advocate: If you started that, the agenda for the policy board meeting would suddenly become domestic.
🪭 Surprising facts about mortgages seen from interest rates
① “Interest received” and “interest paid” are not separate worlds
For depositors, deposit interest is on the income side.
For mortgage users, loan interest is on the expenditure side.
The experience is the exact opposite.
However, neither exists in isolation from the interest rate environment of the financial market.
Therefore, in a phase where interest rates are rising, the benefit for the depositor and the increased burden for the borrower can coexist within the same household as well.
It increases in the right pocket of your wallet, and tension runs high in the left pocket.
I wish you wouldn’t do that in the same pair of pants.
② There is a “time until it transmits” for interest rates
This is quite surprising this time.
After the Bank of Japan raises rates, deposit interest rates, lending rates, and mortgage interest rates do not all switch at the same time.
As a recent track record, the Bank of Japan states that it is normal for ordinary deposit interest rates and the short-term prime rate to change after about two months, new variable mortgages to be later than that, and the application to existing variable mortgages to be even later. Bank of Japan website
Therefore, from the perspective of a consumer, “deposit interest has increased” and “mortgage conditions have changed” arrive as separate pieces of news.
Even though it’s the same wave, the time it takes to reach the shore is different.
③ Just because “interest rates have risen” doesn’t mean you know your repayment amount yet
Mortgages include variable rate types, fixed-rate period selection types, and full-term fixed types. The Japan Housing Finance Agency also has products that combine the full-term fixed Flat 35 with variable rate types and others. Flat 35
Furthermore, even with variable types, you need to check the rules for the base rate, applied rate, and repayment amount revisions depending on the product.
In other words, what you need from the news is
not to stop at
“Mortgage rates have gone up!”
“which interest rate that is, where it applies to your own contract, and when it will be reflected.”
but to look as far as that.
The news is a national broadcast.
Mortgages are individual matters for each household.
🌱 What has become visible now that interest rates have returned
When low interest rates continue for a long time, both depositors and borrowers stop paying much attention to “interest rates.”
However, when interest rates start to move, it becomes easier to see that the same financial environment lies behind things that previously seemed separate, such as bank accounts, mortgages, and corporate borrowing.
Deposit interest rates rise.
The short-term prime rate also moves.
The impact also spreads to mortgages with a time lag.
In fact, the most frequent value for the short-term prime rate of major banks rose from 1.875% in March 2025 to 2.125% in February 2026, and to 2.375% in August of the same year.Bank of Japan website
So, there are only three things to remember this time.
Deposit interest rates and mortgage interest rates do not directly move one another.
There is a time lag in the impact of interest rate hikes.
Mortgages are affected differently depending on whether they are variable or fixed, and on the contract terms.
Within the single phrase “interest rates have risen,” different stories coexist for those who deposit money, those who are about to borrow, and those who have already borrowed.
Watching the same news, one person smiles, while another starts looking for their contract.
Interest rates, they even change the atmosphere in the household.
🎙️ I asked interest rates about those days
*The following is a work of fiction based on the mechanics of interest rates as confirmed. It does not represent actual statements from the Bank of Japan, financial institutions, etc.
🍶 Former bank employee, waiting for the return of fixed deposits: Hey, after making us happy by raising deposit interest rates, is it a bit tone-deaf to have the mortgage side follow right after?
Interest rate: It would be troublesome if you thought I was only in charge of deposits. I am also on the lending side.
🍶 Former bank employee, waiting for the return of fixed deposits: Then why don’t they come all at once? Having the mortgage rate come later is mentally worse.
Interest rate: The mechanisms and timing for reflecting changes in ordinary deposits, the short-term prime rate, new loans, and existing contracts are not the same.
🍶 Former bank employee, waiting for the return of fixed deposits: At least make all mortgage mechanisms the same.
Interest rate: It would be troublesome if you lumped variable and fixed rates together.
🍶 Former bank employee, waiting for the return of fixed deposits: Don’t try to leave with logical arguments. I’m talking about my house here.
🐾 Neko-jiro’s quick interest rate lesson
In the main text, we looked at the ripple effects on deposits and mortgages.
Here, I’ll leave you with just four terms you’re likely to encounter in the news, meow.
Policy interest rate
The short-term interest rate that the Bank of Japan influences in order to manage monetary policy.
When this changes, the impact ripples out to deposit and lending rates through market interest rates and other channels.
Short-term prime rate
An index known as the interest rate used when banks lend to highly creditworthy companies for short periods.
The most frequent rate among major banks became 2.375% in August 2026.Bank of Japan website
However, this does not mean that ‘all mortgages are directly linked to the short-term prime rate’.
Applied interest rate
The interest rate actually applied to your own borrowing.
It is important not to equate the policy interest rates or bank base rates seen in the news with the figures actually applied to your own mortgage.
5-year rule
A mechanism in some variable-rate mortgages that keeps monthly repayment amounts fixed for a certain period.
It is not a rule where the interest rate itself is fixed for 5 years. Also, it is not adopted by all mortgages.Bank of Japan website
Looking at your savings account and feeling happy.
Looking at your mortgage contract and becoming stone-faced.
Only by doing both can you see the full picture of what interest rates are.
🐱 Nekojiro: “I want only the money I receive to fluctuate, meow.”
That is not called monetary policy.
📚 For those who want to know more
What did you think about this topic?
I used to think that when I heard “interest rates are rising,” it just meant one number was going up, meow. When I realized that the way it is communicated and the timing differ for savings, variable mortgages, and fixed-rate mortgages, the way I see the news changed quite a bit, meow.
If you want to dig a little deeper into how mortgages and interest rates work, books related to mortgages and finance are the most natural follow-up to this article, meow. It is easier to follow the story if you look at books that cover not just “what the percentage is now,” but also variable vs. fixed rates and the mechanisms by which interest rates are determined, meow. *Includes affiliate links
*As an Amazon Associate, Ani-Shiron/Nekojiro earns income from qualifying purchases. There is no such convenient monetary policy where reading a book will lower your mortgage interest rate, meow lol