Tokyo's benchmark Nikkei index edged down on Wednesday as investors cashed in on recent gains after a 10-day winning streak.
The Nikkei 225 index lost 0.18% to close at 21 960.71, while the broader Topix index was down 0.49% at 1 606.62.
"Profit-taking prevailed in the market after a tenth straight winning session," said Toshikazu Horiuchi, a broker at IwaiCosmo Securities.
"But stable foreign exchange helped support the bottom line," Horiuchi told AFP.
The dollar fetched ¥108.20 in Asian afternoon trade, slightly up from ¥108.13 in New York late on Tuesday.
Investors remained cautious ahead of the US Fed's policy decision, brokers said.
Observers largely anticipate a rate cut from the Fed, but "the big question for market participants is whether the Fed will commit to more cuts", said Edward Moya, market analyst at Oanda.
The New York Fed on Tuesday pumped billions of dollars into financial markets as a sudden cash shortage drove up interest rates on the eve of the Fed policy decision, which economists widely expect will result in a lower target rate.
"The Federal Reserve Bank of New York was forced to take action on Tuesday," Moya noted, adding that this means the US central bank may have to do more than cutting rates.
Separately, Japan's finance ministry said its politically sensitive trade surplus with the United States edged up 3.8 percent year-on-year in August to ¥472bn ($4.37bn) as imports declined further than exports.
The growth in surplus with the US is the sixth consecutive monthly rise, the ministry said minutes before the opening bell.
The figures come with Washington and Tokyo expected to ink an initial trade agreement later this month in New York.
Japan's overall trade account showed a deficit of ¥136.3bn, the ministry said.
Among individual stocks, Sony dropped 2.21% to ¥6 397 after it said will keep its semiconductor operations, rejecting a proposal by the US activist hedge fund Third Point.
IT and investment giant SoftBank Group was down 1.94% at ¥4 599 but market heavyweight Fast Retailing, the Uniqlo casual wear operator, was up 0.64% at ¥65 260.