The Nikkei broke 65,000 for the first time on Monday, and SoftBank, the listed proxy for both OpenAI and Arm, did much of the lifting.
SoftBank Group shares hit a record high in Tokyo on Monday, carrying the Nikkei 225 above 65,000 for the first time and capping a fortnight in which the Japanese investment conglomerate has added more than $60bn to its market value.
The lift came almost entirely from a single source: growing conviction that OpenAI is days away from filing for an initial public offering that would crystallise the largest single bet Masayoshi Son has ever made.
The Nikkei finished the morning at 65,254, up 3.02% on the session, according to The Japan Times. Easing tension between the United States and Iran, and the prospect that shipping through the Strait of Hormuz will resume, gave the broader market a tailwind.
The SoftBank-specific rally, however, has been running since Thursday, when reports first emerged that OpenAI was preparing to file confidentially for an IPO as soon as the end of last week, targeting an autumn debut at a valuation that could exceed $1tn.
SoftBank trades, in effect, as the only listed proxy for two of the most consequential private positions in the AI build-out: OpenAI, in which it holds roughly 13%, and Arm, the British chip designer in which it retains a controlling stake. Both moved sharply over the past week.
Arm rose more than 16% on Thursday, after a 15% gain the day before, on the back of Nvidia’s earnings beat and the message that hyperscaler spending on AI infrastructure is not slowing. SoftBank, in turn, jumped almost 20% on Thursday and another 12% on Friday, the largest two-day move by a major global company in recent memory.
The numbers behind the OpenAI position have grown to a scale that no longer flatters easy comparison. SoftBank invested $32.4bn in OpenAI during the fiscal year that ended in March, chief financial officer Yoshimitsu Goto told the company’s earnings call this month.
A further $30bn in follow-on investment has been committed, with $10bn funded in April. By October, the cumulative figure will reach $64.6bn. The structure has been underwritten with a $40bn bridge loan from a syndicate now grown to eight banks.
SoftBank booked roughly $45.7bn in cumulative investment gains in fiscal 2025, helping it post net income of $31.4bn, which Goto called the highest profit ever recorded by a Japanese corporation.
Most of the value sits in marks, not in cash; the IPO, if it lands as advertised, is the event that converts the mark to something closer to a price.
There are reasons for caution, and the market mostly knows them. Holding companies typically trade at a discount to the underlying value of their assets, and SoftBank has historically traded at one of the wider ones.
The company’s net asset value, around $300bn in mid-May, swings significantly with movements in Arm and OpenAI on any given week.
OpenAI’s most recent $852bn private round is itself under scrutiny from some of the firm’s own investors, who have flagged the gap between the revenue line and the valuation. Reports that OpenAI fell short of its internal revenue and user-growth estimates have not slowed the rally, but they have not gone away either.
Of the 12 analysts who cover the stock, eight rate it a strong buy, three a hold, and one a strong sell, with an average price target slightly below where the shares are now trading.
The asymmetry, for the next six months, is procedural: the date of the IPO filing, the price range, and the eventual first-day mark.
Son has spent much of the past year arguing, in his characteristic register, that OpenAI is the most important technology shift of the century. Monday’s print is the first time the public market has agreed with him at this scale.
