A strong quake hit western Japan early Monday, but there were no immediate reports of major damage or risk of tsunami waves, officials said.
The 5.3-magnitude quake struck at a depth of 15.4 kilometres (10 miles) at 7:58 am (2258 GMT Sunday) near Osaka, according to the United States Geological Survey.
There was no risk of tsunami from the tremor, the Japanese meteorological agency said, putting its magnitude at 5.9, and the epicentre at a depth of 10 kilometres.
Prime Minister Shinzo Abe told reporters the government was "working united, with its first priority on saving people's lives."
Abe said he had instructed his staff to "swiftly collect information on damage, make utmost efforts in rescuing and saving lives... (and) provide timely and appropriate information to the public."
Local police also told AFP that they had not received reports of substantial damage, and the Nuclear Regulation Authority said it had detected no abnormalities at its facilities after the quake.
The tremor caused a blackout for thousands of houses and suspended train operations during the morning commuting hours.
Private broadcaster TV Asahi showed firefighters responding to a blaze that broke out after the quake at a house north of Osaka city.
Television footage also showed a "shinkansen" bullet train stopped on the railway.
The quake registered a lower six on the Japanese Shindo scale of up to seven, meaning it is hard to stay standing.
Pictures posted on Twitter showed signs at a train station that had fallen down and books thrown off shelves at a store.