The rally has propelled the Tokyo-listed investment giant to a record high, making it Japan's most valuable publicly traded company by market capitalization after it surpassed automaker Toyota Motor on Monday.

The 68-year-old entrepreneur, whose wealth is largely tied to his SoftBank stake, now ranks ahead of Reliance Industries chairman Mukesh Ambani, whose fortune is estimated at $90 billion, according to Forbes' Real-Time Billionaires List.

SoftBank's market value has climbed to about $298 billion, with its shares rising more than 80% this year as investors bet heavily on the company's artificial intelligence strategy.

Masayoshi Son, founder and CEO of investment giant Softbank. Photo from AFP

Son told CNBC on Monday that the AI revolution could create opportunities far greater than those seen during the dot-com era.

"I think this is like more than 10x, probably 50x bigger than dot-com," Son said. "This is the biggest revolution of technology and realization that mankind ever experienced, so this is just like the beginning of the internet."

Investor enthusiasm has been driven by SoftBank's growing position in the global AI race. The company has committed more than $60 billion to ChatGPT creator OpenAI, making it one of the startup's largest backers, while also investing heavily in AI infrastructure projects in the U.S. and Europe.

Its British chip design unit, Arm Holdings, has also benefited from soaring demand for AI computing power, while the group is expanding further into robotics, semiconductors and what Son describes as "physical AI."

According to the company, gains from its OpenAI investment reached about $45 billion over the past year, helping lift annual profit to a record 5 trillion yen (over US$31 billion), the highest in Japanese corporate history, as reported by Financial Review.

Still, some analysts have warned that the rally may be running ahead of fundamentals.

In a research note released Tuesday, Deutsche Bank analyst Peter Milliken said SoftBank's recent gains had been driven by "savvy investing" but also by a market that was showing signs of speculative excess.

"[SoftBank's] been a great run, based mainly on savvy investing, and partly on a bull market that has increasingly shown signals of entering a mania," Milliken wrote, as quoted by Forbes. "Analysts and investors appear to us to have become fixated on short-term momentum, and less interested, or unable, to map out the long-term trajectory with detailed assumptions."