There were no immediate reports of injuries or damage following the minor eruption of the 2,568-metre (8,425 feet) Mount Asama, 150 kilometres (95 miles) northwest of Tokyo.
The volcano last erupted in August last year. Live television footage showed smoke coming from the snow-covered mountain seven hours after the eruption, shortly before 2:00 am (1700 GMT Sunday).
Carried by winds, volcanic ash was seen in towns at the foot of the mountain as well as in the heart of Tokyo, according to the agency.
The agency warned of a possible fall of rocks within four kilometres of the crater.