The No. 3 reactor at the Tomari nuclear plant, on the northern island of Hokkaido, will be shut down for a regular safety inspection, the operator said.
The national government has warned that major cities, including Tokyo and Osaka, face blackouts during the months of peak energy demand – in Japan's notoriously hot and muggy summer – unless local authorities grant permission for utilities to restart the 54 reactors.
Nuclear power previously provided fully one-third of Japan's energy.
But public opinion has turned fiercely against nuclear power since the Great East Japan Earthquake and the massive tsunami that it triggered which destroyed the Fukushima Dai-Ichi nuclear plant, 150 miles north-east of Tokyo.