Kagoshima Prefecture's Mount Io volcano on the island of Satsuma-Iojima erupted at 5:35 p.m. on Saturday, but no injuries were reported, the authorities said.
The Meteorological Agency raised the alert level to 2 on a scale of 5, urging people not to approach the volcano's crater, warning of the likelihood of flying rocks and pyroclastic flows within a 1-kilometer radius of the crater. A plume of ash that spewed from the crater reached over 1,000 meters into the sky, it said.
It was the first eruption of the volcano since June 2013, according to the agency, which said small eruptions could continue for some time.
The agency issued a level-2 alert for the volcano in March 2018 but lowered it to 1 the following month.