The Nuclear Regulation Authority (NRA) said a reactor at a Kyushu Electric Power Co. plant had received approval for construction works upgrading the unit's basic design to meet higher standards set since Fukushima, Sputniknews reported.
Japan's government is aiming to start the first reactor by around June, sources familiar with the plans told Reuters.
That date could be pushed back, however, if courts grant injunctions to prevent restarts of all the country's nuclear power stations, thereby extending Japan's longest stretch without nuclear power since the 1960s.
The NRA's consent on Wednesday was the second in a three-step process that all reactors have to go through before they will be allowed to restart. A final inspection in advance of a restart is also required.
Japan now has 43 operational reactors, down from 54 in March 2011, when an earthquake and tsunami set off meltdowns at Tokyo Electric Power's Fukushima Daiichi plant.
As many as two-thirds of the country's reactors may never return to operation because of high costs, local opposition or seismic risks, a Reuters analysis showed last year.