Tokyo Electric Power Co. said employees ignored an alarm, which led to the overflow and seepage, the Asahi Shimbun reported.
Tepco said the contaminated water did not reach the ocean.
Even though the area is surrounded by a barrier, the water leaked into a rainwater gutter and flowed beyond the barrier, creating pools covering a little over 1,000 square yards, Tepco officials said.
Utility workers ignored an alarm Wednesday that indicated there were dangerously high water levels in the tank, Asahi Shimbun said.
Tepco officials said employees attributed the alarm to malfunctioning instruments because they didn't think water levels were rising because radioactive water wasn't being transferred into the tank.