Mount Asama, which straddles Gunma and Nagano prefectures, erupted around 8 p.m. Wednesday, the Meteorological Agency said.
The agency said the activity level of the 2,568-meter volcano was rated at 2 during the past month. It raised the level to 3 following the eruption.
It said small or midsize eruptions occurred and warned of the possibility of falling rocks on the mountain’s southern slope within 1 km of the crater. The resort town of Karuizawa is on the mountain’s southern slope.
The volcano has shown signs of activity since June 2002 and repeated tiny eruptions occurred between February and April last year.