Residents of the southwestern Japanese prefecture of Oita have been involved in a legal dispute to suspend a nuclear power plant's operation. But a court has turned their demand down.

The case was brought by over 500 residents. The prefecture where they live is separated from the Ikata power plant by a small arm of the sea.

The plaintiffs say the Shikoku Electric Power Company has not taken adequate measures against earthquakes and volcanic eruptions.

They also say the ground could contain problems, including an active fault, but the company has not sufficiently studied the underground structure.

The company says up-to-date safety measures based on the latest scientific knowledge are in place.

The residents are also concerned about the risk of a massive volcanic eruption at Mount Aso, which is about 130 kilometers away. The power company says the risk of that happening while the reactor is running is adequately low.

A district court ruled Thursday that the power company has conducted a reasonable investigation to ascertain the underground structure. It also said the firm's evaluation of the risk of a huge volcanic eruption is also reasonable.

The plaintiffs plan to file an appeal.

This is the first ruling in a series of lawsuits filed by residents in the prefectures of Oita, Ehime, Hiroshima and Yamaguchi. They all want a stop to the Ikata plant's only operating reactor.