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The Nikkei rose to 15,383.91, moving away from a 2-1/2 month low just below 15,000 set on Monday and snapping a four-session losing streak.
But Advantest plunged 4.4% after the maker of chip testing equipment slashed its annual guidance, underscoring the risk of more disappointment as many Japanese companies report earnings over the next few weeks.
The Nikkei is still down 5.6% so far this year, underperforming many of its regional peers.
"This is just an unwinding of risk-off trades. But the rebound is not that strong and it is unlikely to be sustainable," Yasuo Sakuma, portfolio manager at Bayview Asset Management, said of Wednesday's rise.
"Looking at earnings so far, I can say that expectations had been higher (than the actual results). I doubt there will be any boost from earnings."
Traders said buying was driven by short covering by short-term players, while many investors remained cautious ahead of the US Federal Reserve's meeting at which it is widely expected to trim its asset-buying programme by another $10 billion a month.
