Mount Aso, a volcano on Japan's main southern island of Kyushu, erupted on Monday, Japan's Meteorological Agency said, blasting a plume of black smoke 2 km (1.2 miles) high, but there were no immediate reports of damage or injuries.
The 1,592-meter (5,222 feet) Mount Aso is one of the most active peaks in Japan but is also a popular hiking spot. There were a handful of people at a parking lot near the peak but they were being evacuated safely, officials said.
They said the eruption had come without warning.
Kyushu Electric Power said the eruption would have no impact on its nuclear plants, one of which - the Sendai nuclear plant, roughly 160 km (100miles) south of Mount Aso - was restarted in August.
Television footage showed black smoke boiling into the sky and ash was falling as far as 4 km (2.5 miles) away, NHK national television said.
Japan lies on the "Ring of Fire" - a horseshoe-shaped band of fault lines and volcanoes around the edges of the Pacific Ocean - and is home to more than 100 active volcanoes.
Mount Ontake in central Japan erupted unexpectedly last year, killing 63 people, the worst volcanic disaster for nearly 90 years. In May, a remote island south of Kyushu was evacuated due to another eruption.