日経平均が1915円安、AI半導体の売り圧力で米株高の恩恵逃す

On June 16, the Nikkei 225 closed at 66,835 yen, down 1,915 points—a sharp reversal after gaining 1,008 points the prior day to finish at 68,751. The opening session saw a drop of more than 800 yen, falling below the 68,000-yen threshold, and although selling pressure eased in the afternoon, buy-back efforts remained weak. The Tokyo Stock Exchange Prime declined with 446 gainers against 1,070 decliners, with approximate trading volume of 9.56 trillion yen.

The culprit was a collapse in U.S. AI-related semiconductors—SanDisk and Micron fell overnight—which immediately spooked Tokyo investors already sensitive to theme-stock swings. Kioxia HD plunged 15%, becoming Prime's worst performer, while Laser Tek, Advantest, and Tokyo Electron all slid sharply. Related suppliers in AI—Ibiden, SUMCO, and Murata Manufacturing—also posted substantial losses. Because these major index movers weakened simultaneously, retail investors hesitated to buy dips, limiting any rebound.

Yet the sell-off was not universal. Software and SaaS stocks pivoted upward, with NEC, Nomura Research Institute, Free, and Money Forward recording large gains. Among high-turnover names, Toyota, Sony Group, and Nintendo also performed well. In the Growth market, ChatPlus, which had made its debut listing the prior day at 2.1 times its offer price, opened weak on its second day but rallied into a limit-up close on strong follow-on buying.

A secondary shock came via peripheral supply chains. U.S. optical-fiber giant Corning's sudden collapse rippled to Japanese wire and cable makers—Furukawa Electric, Fujikura, and Sumitomo Electric all fell steeply. The body notes that market participants view the critical level as the recent low of 66,268 yen (from July 14). Whether the market can hold that floor on the next session—and whether U.S. AI-semiconductor volatility subsides—will determine Tokyo's near-term direction.

The sharp reversal on June 16 highlights how tightly Japanese equities are now tethered to U.S. semiconductor sentiment. The previous day's 1,008-point rally was driven by a below-forecast U.S. consumer-price reading that eased rate-hike worries, allowing Nvidia and other AI leaders to surge and pull Tokyo's AI cohort higher. Yet within hours, American chipmakers faltered, and that momentum flipped instantly in Tokyo. The body notes that this kind of rapid "material tug-of-war" between markets strengthens swings in theme stocks and tests the resilience of indices that lack broad-based support.

What is striking is that the decline did not crater the entire market. While semiconductor and related-materials leaders crashed in lockstep—Kioxia HD fell 15%, the worst performer on Prime—capital flowed elsewhere. Software and SaaS names rose, and even peripheral plays like cable makers became subject to their own U.S. narratives (Corning's drop pulled down Japanese optical-fiber suppliers). This fragmentation suggests that investors have multiple playbooks at once: AI semiconductors remain the volatility driver, but pockets of resilience exist for those willing to rotate sectors. The body cautions that investors must now discern whether U.S. AI momentum will stabilize, as that condition may be prerequisite for a Tokyo rebound.

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