Smoke spewed from an area near a nuclear reactor at the Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture on Monday night.
Police officials said the smoke may have emanated from a sulfuric acid leak three floors below ground level.
The acid may have leaked from a pipe neutralizing waste liquids from the reactor.
The facility houses the No. 5 reactor -- a boiling water-type light-water reactor -- at the plant, which is operated by Tokyo Electric Power Co. and straddles the towns of Kashiwazaki and Kariwa.
Police officials said there were no signs of any radioactive leakage. The smoke was detected at around 11:25 p.m.
The mishap comes a few months after reports surfaced that Tepco officials falsified reports regarding defects at its nuclear facilities during the 1980s and 1990s.
Earlier this month, Tepco officials said the firm will suspend operations at 16 of its 17 nuclear reactors by the end of March for inspections.