About 15 tonnes of water with a low level of radiation have leaked from the earthquake-damaged Fukushima Daiichi plant.
Water seeped into the ground from a storage tank in the latest of a series of leaks from the Japan nuclear plant, the country's Nuclear and Industrial Safety Agency said.
Plant operator Tokyo Electric Power Co (Tepco) said it was investigating the cause of the leak which was later repaired, as workers struggle to bring under control damaged reactors at the power plant.
Vast amounts of water contaminated with varying levels of radiation have accumulated in storage tanks at the plant after being used to cool reactors damaged when their original cooling systems were knocked out by the March 11 disaster.
Dealing with that radioactive water has been a major problem for Tepco, which is trying to use a decontamination system that cleans water so it can be recycled to cool the reactors.
But the system has encountered technical glitches and officials have said the water could spill into the Pacific Ocean unless the system was operating properly.
The system was halted an hour and a half after it started this week because of a water leakage, however Tepco fixed the problem and rebooted the system the next day.