Shunichi Tanaka, the chairman of the Nuclear Regulation Authority, was speaking at a news conference on Wednesday.
His remarks follow the eruption of Mount Ontake in central Japan on Saturday without any detection of warning signs. Dozens of climbers died in the eruption.
The eruption prompted public concern over volcanic hazard measures for the Sendai plant in Kagoshima Prefecture operated by Kyushu Electric Power Company. There are active volcanoes in the prefecture.
The utility had showed plans to remove nuclear fuel from the facility if it detects any signs of volcanic activity or tectonic plate movement around the plant.
The NRA officially approved the plans last month as part of its screening of measures proposed by the utility based on government requirements for the restart of offline reactors.
At Wednesday's news conference, the NRA chief said Ontake's hydro volcanic eruption is unlike any huge volcanic eruption that may occur around the Sendai nuclear power plant. He says discussing them as the same type of event is not scientific.
Tanaka said that before an enormous eruption, land movements and tremors appear much earlier than for the ongoing eruption of Mount Ontake.
The NRA has held 2 meetings with volcanologists since August. Participants pointed out the difficulty of predicting volcanic eruptions using current knowledge and technology.