Columns of thick white steam were rising from the 3,067-metre (10,121-foot) Mount Ontake, which erupted around noon on Saturday, spewing ash, rocks and steam on otherwise a sunny autumn weekend busy with tourists and hikers.
Seven people were unconscious and buried under ash, national broadcaster NHK said.
One of them was rescued but the others remained on the mountain, NHK said, adding that a total of 42 people were believed to have suffered injuries.
Local media on Saturday reported that a person died, but firefighters said the death had not been confirmed.
A suffocating blanket of ash up to 20 centimetres (eight inches) deep covered a large area of the volcano, trapping climbers and forcing up to 150 to seek refuge in mountaintop shelters at one point.