October 15, 2026

Why did the Nikkei Stock Average fall due to US prices? – The connection between “prices, interest rates, and AI stocks” that even elementary school students can understand …

On October 15, the Nikkei Stock Average (a figure summarizing the stock prices of 225 representative Japanese companies) fell significantly from the morning. During trading, it dropped by more than 2,300 yen compared to the previous day. The trigger was the price (cost of goods and services) figures announced in the US the previous night. US prices in September were 3.7% higher than a year ago, a result higher than expected. When prices rise, interest rates (the rental fee for borrowing money) become harder to lower, and stocks of AI and semiconductor companies are sold off. That trend reached Tokyo across the ocean.

This article proceeds in the order of “this happened -> let’s learn about this today.”

This is what happened – From the announcement of US prices to the sharp drop in the Nikkei Stock Average

On the night of October 14, US prices for September were announced, and the figures were higher than expected. US stocks fell that day, and the next morning, the Nikkei Stock Average also fell significantly.

Arranged in chronological order, it looks like this.

October 14, 9:30 PM (Japan time): The US government announced September prices
October 15, 5:00 AM (Japan time): US stock trading ended. The S&P 500, a figure summarizing stock prices, fell 1.11%, the NASDAQ, which has many high-tech companies, fell 1.88%, and the figure summarizing semiconductor company stocks fell 4.30%
October 15, 9:00 AM: Tokyo stock trading began, and the Nikkei Stock Average opened significantly lower

Japanese newspapers report the reason for the drop in Tokyo as follows: the flow is that prices in the US the previous day exceeded expectations, and because of concerns that prices and interest rates would rise, stocks of AI and semiconductor companies were sold off.

First of all, where is this about? – The US and Japan

This story is about two countries: the US, which announced the prices, and Japan, where that movement reached.

The meanings of the colors on the map are as follows.

United States (red): The country with the largest economy in the world. The capital is Washington, and it is home to the government office that announced the prices and the FRB (Federal Reserve Board). The FRB is the US central bank (a government office like the boss of banks that decides the amount of money in the country and interest rates). The center of stock trading is New York
Japan (blue): Our country. We trade stocks in Tokyo

The US and Japan are separated by the ocean and have a 13-hour time difference (in the current season). Because of this time difference, news from the US at night reaches the stock market in Japan in the morning.

Why do stocks fall due to US prices? – 3 connections

When US prices rise, more people think that the FRB might continue to raise interest rates. When interest rates are likely to rise, stocks that were bought in anticipation of future profits are sold off.

First: Prices rose more than expected

The US Consumer Price Index for September (a figure summarizing the prices of everyday shopping) rose 3.7% from a year ago.Compared to the previous month, it also rose 0.6%, which was a larger increase than the 0.4% in August.

More than half of the increase was due to the price of gasoline. Gasoline is 9.1% more expensive than the previous month. The Nihon Keizai Shimbun reports that the rise in gasoline and diesel prices due to turmoil in the Middle East pushed it up.

Prices excluding food and energy, which have large price fluctuations (called core prices), also rose 2.6% from a year ago. US media reported that this core figure also exceeded expectations.

Review: Why do interest rates rise when prices rise?

The relationship between prices and interest rates is the “accelerator” and “brake” story we saw in the fourth installment of this series. Central banks step on the “accelerator” of lowering interest rates when the economy cools down, and step on the “brake” of raising interest rates when the economy is overheating and prices are likely to rise rapidly.

When interest rates rise, it becomes harder to borrow money, and the money circulating for shopping and corporate equipment decreases. This cools down the heat of the economy and stabilizes the way prices rise. In other words, the more prices are rising too much, the more the central bank wants to raise the brake interest rates.

In the fourth installment, we looked at this mechanism through the interest rates of the Bank of Japan (BOJ), Japan’s central bank, and the topics of home loans and deposits. If you want to know more, please click here.

Second: The FRB might continue to raise interest rates

In the US this time, the FRB is the one stepping on the brake. The FRB aims to keep the rate of price increases at around 2% per year. On September 16, they raised the policy interest rate (the interest rate that serves as the basis for the rental fee for money, determined by the FRB) by 0.25% to 3.75-4%. The announcement at that time also included a sentence stating that “the rate of price increases is still high.” In other words, as of September, the FRB viewed that prices had not yet settled down to their 2% target.

Then, price figures higher than expected were released. Reports indicate that the view has strengthened that the FRB will raise interest rates again at the next meeting on October 27-28. After the announcement, the interest rate on 10-year US Treasury bonds (certificates of national debt) temporarily rose to the 5.27% range.

Third: Stocks expected to have future earnings were sold off

When interest rates rise, stocks bought in anticipation of future earnings are more likely to fall in price. Securities companies also explain that when interest rates rise, “the stock prices of growth stocks (stocks of companies expected to earn significantly in the future) are more likely to fall sharply.” In other words, the more a stock is bought in anticipation of distant future earnings, the more likely it is to fall in price when interest rates rise.

AI and semiconductor companies have had their stocks bought not for current earnings, but for the expectation that “they will earn more with AI in the future.” That is why they are the first to be sold off when there are concerns about interest rates. This is why the figure summarizing the stocks of semiconductor companies fell 4.30% on October 14.

On the same day, the price of corporate bonds (certificates issued when a company borrows money) of CoreWeave, a company that rents out AI computers, also fell sharply. The Nikkei reported that the reason for this was the size of the company’s debt (about $52 billion at the end of June) and the increase in interest payments (rental fees paid on borrowed money). CoreWeave’s stock ended down 15%. For companies that use debt to acquire AI computers, if interest rates rise, their interest payments increase accordingly.

Thinking in terms of “future allowance vouchers”

Let’s compare the third connection (when interest rates rise, stocks expected to have future earnings are sold off) to “future allowance vouchers.”

A friend is selling “vouchers that will give you 10,000 yen in 10 years.” When bank deposits barely increased, these vouchers were popular with everyone and sold at high prices.

However, bank interest rates have risen. Now, if you deposit money in a bank, it will grow significantly in 10 years. As a result, more people think, “Maybe it’s better to deposit in a bank than to wait 10 years for a voucher,” and the price of the vouchers goes down.

The items in the analogy correspond to the following items in the main text.

The 10-year allowance voucher corresponds to stocks bought in anticipation of future earnings (AI and semiconductor companies) (third connection)
Bank interest corresponds to interest rates (second connection)
The news that “interest rates are likely to rise” corresponds to price figures higher than expected (first connection)

For us in Japan – The Nikkei Stock Average and the US within All Country

The flow of semiconductor and AI stocks sold in the US reached Japanese stocks the next morning.

The Nikkei Stock Average also fell significantly

On the morning of October 15, the Nikkei Stock Average opened significantly lower, and during trading, it fell more than 2,300 yen below the previous day’s closing price (67,744.29 yen). The Nihon Keizai Shimbun reported this with the headline “Expectations of US interest rate hikes are a burden.” In other words, the view that interest rates in the US are likely to rise again pushed down Japanese stocks.

There are many large companies involved in semiconductors and AI in the Japanese stock market as well. The next morning, after the index of semiconductor company stocks in the US fell by 4.30%, the same concern reached Tokyo.

The US within All Country and AI

People who are investing in All Country (the investment trust “eMAXIS Slim Worldwide Equity (All Country)” that invests in stocks around the world) through NISA might think, “What does the story about US prices have to do with my All Country?”

In the September 30 report for All Country, the breakdown by country was 63.4% for the US and 5.2% for Japan. Looking at the types of companies, “Information Technology,” such as PCs and semiconductors, was the largest at 32.4%, accounting for about 3,240 yen out of every 10,000 yen. The remaining 6,760 yen is invested in other types of companies, such as banks and factories.

Among the top 10 companies, these 6 are involved in AI or semiconductors.

NVIDIA: 5.0%
Apple: 4.6%
Microsoft: 3.4%
TSMC: 1.8%
Broadcom: 1.5%
Micron: 1.1%

Although All Country invests in a diversified manner around the world, about one-third of its contents are high-tech companies that are easily sold off due to interest rate concerns.The US price figures are a matter that also reaches the price of All Country.

From an investment perspective – Tailwinds and headwinds when interest rates are unlikely to fall

When interest rates are unlikely to fall or are likely to rise, it becomes easier for headwinds to blow against companies expected to grow in the future and companies with high debt, and for tailwinds to blow for those depositing money to receive interest.

Headwind: Companies bought in anticipation of future profits. Stock prices are more likely to fall.
Headwind: Companies that have borrowed money and are spending large amounts. Interest payments increase.
Tailwind: Those who deposit money and receive interest. Interest on savings and the like is likely to increase.

However, this current discussion is based on the view that they “might raise rates at the next meeting.” It has not yet been decided whether the Fed will actually raise interest rates on October 27-28. Therefore, when it comes to price news, “it is easier to avoid misreading if you separate whether interest rates actually rose or if it was sold off due to concerns that they might rise.”

*This does not recommend any specific stocks (individual companies whose shares can be bought and sold).

I have placed materials summarizing this way of thinking about “how to protect your money” at the end of the article.

Today’s Term – “Consumer Price Index”

The Consumer Price Index is a figure that measures how much the prices of goods and services we buy have changed on average. It is announced every month.

How it is measured: We put everyday purchases like food, gasoline, rent, and clothing into a single basket and see how much the price of that basket has changed compared to a year ago.
Core: This is the figure excluding food and energy, which have large price fluctuations. It is used to see the trend of prices.
This time: US consumer prices in September rose 3.7% from a year ago, and the core rose 2.6%.

Because the consumer price figures came in higher than expected, the view that the Fed will continue to raise interest rates strengthened, and AI and semiconductor stocks were sold off.

A shopping basket that cost 1,000 yen last year cost 1,037 yen this year. This is what “rose 3.7%” means. The Fed wants to settle this down to around 1,020 yen (2%).

In Japan, the Ministry of Internal Affairs and Communications also releases a similar Consumer Price Index every month. If you hear “prices” on the news, it becomes easier to understand if you think of it as the price of this shopping basket.

Frequently Asked Questions

How much did the Nikkei Stock Average fall on the morning of October 15?

In trading on the morning of October 15, it fell by more than 2,300 yen from the previous day’s closing price (67,744.29 yen). Prices were announced in the US the previous night, and the trend of falling US stocks reached Tokyo directly in the morning.

Why do Japanese stocks fall because of US prices?

When US prices rise more than expected, the view that the US central bank, the Fed, will continue to raise interest rates strengthens. When interest rates are expected to rise, AI and semiconductor stocks, which are bought in anticipation of future profits, are sold off. That trend reached Tokyo the next morning. Japanese newspapers also cited the previous day’s US price figures as the reason for the decline.

How much did US prices rise in September?

They rose 3.7% from a year ago. The core, excluding food and energy, was 2.6%. More than half of the increase was due to the price of gasoline (9.1% higher than the previous month).

Will the Fed really raise interest rates?

It has not been decided yet. The Fed just raised interest rates on September 16, and the next meeting is October 27-28. It is reported that the view that they will raise them again after the price announcement has strengthened.

What kind of company is CoreWeave?

It is an American company that rents out AI computers to other companies. They have taken on a lot of debt to acquire these computers, and on October 14, the price of the company’s corporate bonds (debt certificates) dropped sharply, and its stock also fell by 15%.

I am investing in All Country (Orkan) through a savings plan. How should I think about this?

All Country is composed of 63.4% US stocks and 32.4% information technology companies (as of September 30). Knowing that its contents are prone to price fluctuations based on interest rate news will help you stay calm on days when prices drop suddenly. Since you are investing in stocks from all over the world, it is important to use this as material to consider whether to continue your savings plan over the long term, rather than focusing on daily movements.

Why do stock prices fall when prices are rising?

When prices rise, the sales figures of companies may also increase. However, if prices rise too much, the Fed will raise interest rates to try to stabilize them. When interest rates rise, it becomes harder to borrow money, and stock prices that are based on future expectations tend to fall. The stock market moves by looking at these future trends, such as the expectation that “interest rates are likely to rise.”

Conclusion — The news is about your own money

The price figures released by the US government at night caused US stocks to fall that day, and by the next morning, the impact reached Japanese stocks and the AI companies within All Country.In the first and second installments, we looked at how Middle Eastern crude oil, in the third, AI money, in the fourth, interest rates (the rental fee for money), and in the fifth, South Korean semiconductors reached your own money. What we looked at this time is how the “price of a shopping basket” in the US reaches Japanese stocks through interest rates.

The theme of this series is to take up one thing that has happened in the world at a time and decipher it with words that even elementary school students can understand, saying “Let’s learn this today.”

How to protect your money so you are not swayed by global movements is summarized here. You can receive three things: a PDF manual, a video seminar, and priority notification for free consultations, via our official LINE account.