Why does SoftBank Group's stock price move by 10% on just 'OpenAI rumors'?—Verifying a year's worth of price movements
Today’s price movement
SoftBank Group (9984, Tokyo Stock Exchange Prime) closed at 6,430 yen on September 30, up 6.55% from the previous day (6,035 yen → 6,430 yen). The trigger was reports that OpenAI is considering a new funding round at a valuation of $1.4 trillion to raise $30 billion. SoftBank Group is a major investor in OpenAI, and since OpenAI itself is private, there is a structure where investors gain exposure to fluctuations in OpenAI’s corporate value through ‘SoftBank Group’s stock price’.
This pattern of ‘OpenAI speculation reports → SoftBank Group stock price moving by several percent to over 10%’ is actually not the first time this has happened. When comparing the past year’s stock prices with TDnet/EDINET disclosures and news reports, the same structural price movements can be confirmed repeatedly.
Why does SoftBank Group’s stock price move on ‘OpenAI funding’?
A question might arise here. It is OpenAI that is raising funds, not SoftBank Group itself. Why does the stock price of SoftBank Group move just because of that report?
The point is that SoftBank Group has a character closer to an ‘investment holding company’ than an ‘operating company.’ Much of the company’s corporate value is explained by the sum of the valuation of the shares and equity it holds (so-called NAV, or Net Asset Value), such as Arm, OpenAI, and Vision Fund portfolio companies. It is a common view that the market evaluates SoftBank Group’s stock price by accumulating this NAV and then applying a certain ‘discount’ due to the complexity of management and caution regarding past high-risk investment decisions (the so-called ‘SoftBank discount’).
What comes into play here is the fact that OpenAI is a private company. If it were a listed company, it would have a stock price every day, but for private companies, there is no such thing as ‘today’s market price.’ Therefore, investors (and SoftBank Group’s own fair value assessment in its financial results) use the price set in the most recent funding round as a proxy for the company’s ‘current market value.’ This is an evaluation mechanism common to all private startups, not just OpenAI.
According to reports as of March 2026, SoftBank Group’s cumulative investment in OpenAI is expected to reach $64.6 billion, with an equity stake of approximately 13%. In other words, if OpenAI conducts a new funding round at a ‘valuation of $1.4 trillion,’ that figure becomes the new reference point for calculating the valuation of SoftBank Group’s holdings. A simple calculation would be 13% × $1.4 trillion = approximately $182 billion, but this is merely a rough estimate (both the equity ratio and the reference valuation can change based on subsequent reports and actual funding terms). The important thing is the structure itself: ‘the fact that the reference market value of held assets increases works to push up SoftBank Group’s NAV.’
Furthermore, the fact that a large-scale funding round has actually been completed also serves as a signal that ‘there are still investors who want to invest in OpenAI at a high valuation.’ By reducing future uncertainty, it is also expected that the aforementioned ‘discount’ itself will shrink slightly. In other words, it can be organized as both (1) the effect of the valuation of the held assets themselves increasing, and (2) the effect of the market’s discount rate on that valuation decreasing, both acting as factors to push up the stock price.
Please note that the figures of 13% and $64.6 billion are based on reports as of March 2026 and may have changed due to subsequent additional investments or changes in terms.
Chronology of major price movements over the past year
The following are major price movements (±5% or more in a single day or over multiple days) where factors could be identified by cross-referencing stock price data (based on adjusted closing prices) and news reports.
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January 22, 2026 +11.6%(3,875 yen → 4,325 yen): The rise in shares of the UK’s Arm Holdings, which it holds, acted as a tailwind. Unrealized gains on Arm shares are one of the major variables in SoftBank Group’s corporate valuation.
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May 21, 2026 +19.8%(5,039 yen → 6,039 yen): US Nvidia’s earnings exceeded market expectations, reigniting optimism for AI-related stocks as a whole.
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May 22, 2026 +11.9%(6,039 yen → 6,757 yen): The trend from the previous day continued, resulting in a sharp rise of +34% over two business days.
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June 26, 2026 -12.5%(7,118 yen → 6,226 yen): The New York Times reported that ‘OpenAI is considering delaying its IPO until 2027.’ On the same day, the Nikkei Stock Average also fell by over 4% in sympathy.
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September 15, 2026 +7.5%(5,839 yen → 6,279 yen): Short-covering buying occurred following the rise in US semiconductor stocks.
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September 30, 2026 (Today) +6.55%(6,035 yen → 6,430 yen): As mentioned at the beginning, reports of OpenAI considering new funding.
When laid out like this, the axis that divides the direction of price movements becomes visible. The upward phases are all ‘factors that increase the valuation of held assets (Arm, OpenAI),’ while the downward phases are ‘factors that create uncertainty regarding OpenAI’s IPO or fundraising plans.’ In other words, SoftBank Group stock has the characteristic that the majority of its short-term price movements are explained by speculation regarding the valuation of its held private AI assets, rather than the performance of the business itself.
What could move the stock price going forward?
Based on past price movement patterns, I will organize specific candidate triggers for each direction.
Upside direction
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If the reported $30 billion fundraising is actually finalized and officially announced, there are historical precedents for double-digit percentage gains through the revaluation of held assets, such as the Arm stock surge on January 22 (+11.6%) and the post-Nvidia earnings surge in May (+34% over two days).
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If a major U.S. semiconductor company’s earnings result in a positive surprise, there is a possibility that the pattern of renewed optimism involving the entire sector, similar to September 15 and May 21, could be repeated.
Downside direction
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If negative reports regarding OpenAI’s IPO plans emerge again (such as delays or withdrawals), a sharp decline pattern similar to June 26 (-12.5%) could occur. At that time, the Nikkei Stock Average itself also fell in sympathy, and such events are prone to spreading to the entire index.
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If the sentiment of the entire semiconductor and AI-related sector reverses (the reverse pattern of buying factors like on September 15), SoftBank Group is likely to move in tandem through its held assets (such as Arm stock).
Please keep in mind that none of these are ‘confirmed plans,’ but rather an organization of possibilities based on price movement patterns observed over the past year.
Sources
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Nihon Keizai Shimbun, Bloomberg, The Standard (HK), Reuters-related reports
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IRBANK stock price data (get_price_history, sourced from TSE)
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IRBANK disclosure information (list_disclosures, sourced from EDINET/TDnet)
This is not investment advice, but research based on publicly available information.