Defence officials said six soldiers, who were among 30 conducting ski training, were buried by an avalanche about half an hour after the eruption, but they and the seventh skier had all been pulled out of the snow.
The falling rocks kicked clouds of snow into the air as they made impact.
"About 100 people have evacuated", he said.
Japan's Meteorological Agency urged nearby residents to stay away from Mt. Kusatsu Shirane after it detected what it said was "slight volcanic activity". It is one of 50 mountains continuously monitored by the weather agency.
Around 15 people were injured as a result of the eruption, local firefighters said.
At least 14 people have also been injured, several of them critically. Among the injured were five SDF personnel, said a spokesman for the forces.
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One skier was reported missing following the incident that began after the volcano exploded.
As per reports on state broadcaster NHK, the debris from the volcano fell nearly 1 km from the mountain as rocks came crashing downwards, injuring the skiers present at the ski resort.
The gondola has been stopped because of a power outage.
A military helicopter airlifted the first group of eight skiers to safer ground, according to footage shown by NHK public television.
Japan, with scores of active volcanoes, sits on the so-called Pacific "Ring of Fire" where a large proportion of the world's quakes and volcanic eruptions are recorded.
The most powerful explosion in recent years was the 1991 eruption of Mount Pinatubo, about 100 kilometres northwest of Manila, which killed more than 800 people.