The Nuclear Regulation Authority said a worker conducting system inspections mistakenly pushed a button turning off power to some of the systems in the four reactor buildings at the Fukushima plant.
Plant operator Tepco pours hundreds of tonnes of water a day over the reactors to keep them cool after a devastating earthquake and tsunami in March 2011 triggered meltdowns and hydrogen explosions.
Tepco said water was being pumped to the No 1, No 2 and No 3 reactors at the plant and pools storing spent fuel rods were being cooled.
The latest incident is another reminder of the precarious state of the Fukushima Daicchi plant, which has suffered a series of mishaps and accidents this year.
In March, Tepco lost power to cool spent uranium fuel rods at the plant after a rat tripped an electrical circuit.