Japan's stock market has had one of the most emphatic runs of any major market in 2026, and the numbers are genuinely hard to overstate: the Nikkei 225 is up nearly 33% year-to-date, hit record highs for a third straight month in June, and posted a quarterly gain of 37% between April and June — the largest quarterly increase since records began in 1965. It even opened the year with the strongest start for both the Nikkei and the broader Topix index since 1990. None of that has come without turbulence: concerns about AI infrastructure overinvestment have driven sharp, sudden swings even within a generally rising market, including a week in July that saw an early plunge followed by a rapid rebound.
The short version: Nikkei 225 up ~33% year-to-date in 2026; record highs for a third straight month in June; Q2 (April-June) gain of 37%, the largest quarterly increase since 1965 records began. The index closed at 62,833.84 on May 7 after its largest single-day point gain in history (3,320 points), and later rose above 68,000 for the first time. Drivers include a genuine AI-driven buying frenzy, government stimulus targeting strategic industries, and a gradual Bank of Japan shift away from ultra-loose policy that's being read as confidence rather than alarm. "AI overinvestment" concerns are causing real volatility within the broader uptrend.
Why rate hikes haven't derailed the rally
Rising interest rates typically pressure stock valuations, so a market rallying this hard while its central bank tightens policy is worth explaining rather than just noting. The Bank of Japan has been gradually moving away from decades of ultra-loose, deflation-era monetary policy through 2026, and investors appear to be reading each hike as a sign the central bank finally trusts the economy to handle higher rates — a fundamentally different signal than an emergency hike aimed at stopping runaway inflation, which is the scenario that typically does spook equity markets. That reframing — tightening as confidence rather than alarm — has been a real, if unusual, feature of this year's rally.
The AI overinvestment worry, and why it causes swings without ending the trend
Concerns over excessive investment in AI infrastructure — chips, data centers, and related buildout — have weighed on sentiment at points in 2026, contributing to genuinely volatile trading, including a July stretch where the market swung sharply from an early-week plunge to a rapid rebound. The underlying worry is straightforward: capital is flowing into AI infrastructure faster than near-term revenue clearly justifies, which is the same tension playing out across AI-heavy markets globally, not something specific to Japan. It's been enough to cause sharp short-term moves without derailing the broader uptrend so far.
Berkshire Hathaway's Japan bet, and what it signals
Warren Buffett's Berkshire Hathaway has built and increased substantial stakes in several of Japan's major trading companies (the sogo shosha) over recent years, and that positioning has been widely cited as a factor helping validate broader international investor confidence in Japanese equities. It's a data point about sentiment, not a specific stock recommendation from this article — but it's part of why global capital has kept flowing toward Japan even as valuations have climbed through 2026's rally.
Advantest: the AI chip-test stock at the center of the rebound
When Japanese tech stocks rebounded on June 30, 2026 — a session that helped drive that record 37% quarterly gain — the leaders were telling: Advantest rose 8.13%, alongside Toho Zinc (+7.48%) and Sumitomo Metal Mining (+7.17%). Advantest makes semiconductor test equipment — the machinery that verifies chips actually work before they ship — which makes it a direct proxy for AI chip demand without being a chipmaker itself. That positioning has made it one of the most closely watched Japanese stocks in the AI trade, since its order book offers a read on chip-industry demand somewhat earlier in the production cycle than the chipmakers' own results do.
| Metric | 2026 figure |
|---|---|
| Nikkei 225 YTD | +~33% |
| Q2 2026 (Apr-Jun) quarterly gain | +37% (largest since 1965 records began) |
| Largest single-day point gain | +3,320 points (May 7, 2026) |
| Advantest (June 30 rebound session) | +8.13% |
What this means if you're watching from outside Japan
- A central bank hiking rates isn't automatically bearish for stocks — context (why it's hiking) determines how markets read it, as Japan's 2026 rally shows.
- Sharp short-term swings within a strong uptrend are a feature of AI-heavy markets generally, not unique to Japan — the same overinvestment debate is playing out in the US and China too.
- Equipment and supply-chain stocks (like Advantest) can be a useful early read on AI chip demand, distinct from betting directly on the chipmakers themselves.