Tokyo Electric Power Company says workers at the crippled Fukushima Daiichi nuclear power plant have found that untreated water leaked from the outdoor vent of a filtering device.
The plant operator said the water, some of which may have seeped into the ground, is believed to have contained 220 times the standard level of radioactive substances that has to be reported to the government.
TEPCO said the leak was detected shortly before 9:00 a.m. on Wednesday.
The utility says the outflow from the vent may have occurred because a valve that was supposed to be shut remained open when workers washed away untreated water inside the filtering device during an inspection.
The leak stopped when the flow of water inside the device was halted.
But Tokyo Electric estimates that about 5.5 tons of water got out.
It also estimates that the leaked water contained about 22 billion becquerels of cesium-137 and other radioactive substances that emit gamma rays.
The standard level for reporting to the government is 100 million becquerels.
The utility says it has not confirmed any environmental impact outside the plant.
But because the leaked water may have seeped into the ground, it plans to recover the water and surrounding soil.