TOKYO — Japan’s second-largest nuclear operator said Saturday that it was manually shutting down a reactor in central Japan after a technical malfunction.

No radiation had leaked from the No. 1 Reactor at the Oi Nuclear Power Plant, on the Japan Sea coast, about 250 miles west of Tokyo, said Yoshihiko Kondo, a spokesman for the plant’s operator, Kansai Electric Power.

Mr. Kondo said that a loss of pressure had been detected late Friday in an accumulator tank needed to cool the reactor core in an emergency, forcing the utility to shut down the reactor. Workers will begin the shutdown at 1 p.m. Saturday in Japan, and shutdown will be complete by 9 p.m., he said.

The shutdown at Oi just four months after the devastating accident at the Fukushima Daiichi Nuclear Power Plant comes as a blow to an industry struggling to regain public confidence. It also worsens an electricity shortage that has forced several utilities in Japan to require companies to reduce their use by at least 15 percent.