A strong 6.2 magnitude earthquake struck central Japan late Saturday, the US Geological Survey said, with Japanese media reporting at least five people were feared trapped under collapsed houses.
The quake struck at 1308 GMT, with its epicentre at a depth of 10 kilometres (six miles) in the north of Nagano Prefecture, northwest of Tokyo, according to USGS.
Japan Meteorological Agency measured the quake at magnitude 6.8.
The quake toppled several houses in Nagano, with broadcaster NHK saying rescuers feared five people were trapped under the rubble.
There was no immediate confirmation of casualties as police and municipal officials said they were scrambling to collect information.
"We are trying to confirm any casualties or damage," a spokesman at the Nagano prefectural police told AFP by telephone.
There was no damage to any of the seven nuclear reactors at the sprawling Kashiwazaki-Kariwa plant in neighbouring Niigata prefecture as they have been off-line since 2011, NHK quoted the operator, Tokyo Electric Power Co., as saying.
Japan is hit by around a fifth of the world's powerful quakes every year and sits at the conjunction of several tectonic plates.
A strong tremor revives memories of the 9.0 earthquake in March 2011, which triggered a tsunami which sparked the Fukushima atomic disaster and left some 18,000 people dead or missing.