Mount Aso, which stands 1,592 meters (somewhat more than 5,200 feet) tall, is in Kumamoto Prefecture on the southwestern island of Kyushu.
Pictures from the zone demonstrated a layer of volcanic fiery remains covering roads, rooftops and vehicles. No wounds have been accounted for, the Japan Meteorological Agency said.
It raised the ready level for the region to 3, with 5 being the largest amount, and cautioned individuals from the general population not to approach Mount Aso and to be careful with falling rocks.
"It is greatly uncommon for an ejection section to surpass 10,000 meters," Makoto Saito, chief of the organization's Volcanology Division, told CNN subsidiary TV Asahi on Saturday.
Saito included that while the fountain of liquid magma is in its dynamic stage, there is a "plausibility" another ejection of the same scale could happen.
Mount Aso already thundered into life in September a year ago.
Japan, which sits on the Pacific "ring of flame", has scores of dynamic volcanoes. Level 3 alarms have likewise been issued this year for the Sakurajima and Kuchinoerabujima volcanoes, as indicated by the Japan Meteorological Agency.