Tokyo: Mount Aso in Japan erupted at 9.43 am (local time) on Monday, the Meteorological Agency said.
The eruption launched a thick black plume of smoke into the air and forced dozens of tourists to evacuate, Japan Times reported.
It was the first time the weather agency issued an eruption warning since it began using a new bulletin system in August.
The weather agency upgraded the alert level from two to three, which prohibits people from entering the mountain.
As of 10 am (local time) there had been no reports of casualties, Kumamoto prefecture 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.
Last year, Mount Ontake of Japan erupted unexpectedly killing 63 - Japan`s worst volcanic disaster in nearly 90 years.