Eight Japanese soldiers were among those struck by rocks while ski-ing on the slopes of Mount Kusatsu Shirane, the Ministry of Defence said in a statement.
One of the soldiers later died. About a dozen people were injured during the eruption and avalanche near a popular mountain resort in Gunma in central Japan on Tuesday morning.
Many were apparently hit by volcanic rocks on Mount Kusatsu Shiran with two were critically injured and three seriously, national broadcaster NHK said.
The Japanese Meteorological Agency (JMA) warned that further eruptions could not be ruled out and that rocks could be thrown as far as a mile from the peak.
The warning level for the peak was raised to 3, meaning that people should not climb the 2,160m (7,090 ft) volcano.
Japan has 110 active volcanoes and sits on the so-called Pacific ‘Ring of Fire’ which is the world’s most active earthquake belt.
Video footage from the top of the resort’s gondola showed skiers gliding down the slopes as black rocks plummeted from the skies and snow billowed up as they struck the ground, sometimes just missing skiers. A cloud of black smoke later drifted in.
“There was this huge boom, and a big plume of totally black smoke rose up,” one skier told NHK. “I had absolutely no idea what had happened.”
A photograph taken at the site and shown on NHK depicted a gondola with a shattered window. At least several of the injuries were due to broken glass.
“Other people appeared to be hurt by the stones, which appeared to be around 10 to 20 cm in size,” another skier told NHK.
The resort temporarily lost power, leaving a number of skiers suspended in gondolas for around half an hour until they resumed moving. Around 80 skiers at a hut at the top of the mountain were awaiting rescue, which began by helicopter on Tuesday afternoon.
In September 2014, 63 people were killed on Mount Ontake, the worst volcanic disaster in Japan for nearly 90 years.