A man looks at a stock quotation board displaying the Nikkei share average outside a brokerage in Tokyo, Japan, October 21, 2025. REUTERS/Manami Yamada

By Rocky Swift

TOKYO (Reuters) -Japan's Nikkei share average surged past the key 51,000 level for the first time on Wednesday, underpinned by investor optimism over artificial intelligence sector investments.

The tech-heavy Nikkei 225 Index was up 2.05% at an unprecedented 51,249.82, as of the midday break. The index crossed the key psychological barrier of 50,000 on Monday.

The broader Topix index edged up less than 0.1%, as a stronger yen weighed on most non-tech shares.

Advantest, a key chip-testing equipment supplier to major technology companies, soared more than 20% after the firm hiked its annual profit forecast by one quarter on surging demand.

"Stocks such as Advantest and SoftBank Group are essentially driving the stock market," said Nomura Securities strategist Wataru Akiyama. "It's unclear how long this will last. Once the initial wave passes, the upward momentum may ease."

All three major U.S. stock indexes closed at record highs after AI darling Nvidia said it will build artificial intelligence supercomputers for the U.S. energy department.

Shares of Microsoft jumped after the tech company reached a deal that permits OpenAI to restructure into a public benefit corporation while giving Microsoft a 27% stake in the ChatGPT maker.

Japan's SoftBank Group, a major investor in chips and AI, climbed 2.1% and was the second-biggest driver of the Nikkei in terms of index points. Lasertec, another semiconductor heavyweight, surged 7.8%.

There were 65 advancers in the Nikkei against 160 decliners. The largest losers were Nidec, which lost another 7.3% after being marked for exclusion in the Nikkei, followed by SHIFT, down 5.3%.

(Reporting by Rocky Swift in Tokyo; Editing by Sherry Jacob-Phillips)