The 1,592-metre (5,222ft) Mount Aso, popular among tourists for hiking, is one of the most active volcanoes in the country.
There were no immediate reports of casualties and damages. There were a handful of people at a parking lot near the peak but they were being evacuated safely, officials said.
The volcano eruption has prompted cancellation of several flights.
Mount Ontake in central Japan erupted unexpectedly last year, killing 63 people, the worst volcanic disaster for nearly 90 years.