A Japanese soldier was killed when a volcano erupted, triggering an avalanche and sending rocks raining down on a kilometre-wide area.
At least 11 people were injured, some of them critically, rescue services said.
Six of those trapped were members of Japan's Ground Self Defence Force (SDF), according to Defence Minister Itsunori Onodera.
All were rescued but most were injured and one of them later died, the Defence Ministry said.
One person was trapped in the avalanche for some time before rescuers managed to dig them free.
National broadcaster NHK said at least 17 people were injured, with many apparently hit by volcanic rocks.
“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.”
Another witness told the agency they were evacuated within 30 minutes of seeing the smoke rise from the top of the mountain.
Kusatsu-Shirane, a 2,160-metre (7,090 foot) volcano, erupted on Tuesday morning, the Japanese Meteorological Agency (JMA) said.
Further eruptions could not be ruled out and rocks could be thrown as far as 2 km (1 mile) from the peak, the agency warned.
Video footage from resort showed skiers travelling down the slopes as black stones fall from the sky.
The force of the stones falling sends clouds of white snow upwards. Meanwhile, a thick column of black smoke drifts up.
A number of those injured were hurt by the resorts windows shattering.
Around 80 skiers were still awaiting rescue on the mountain on Tuesday afternoon.
It was unclear whether the avalanche was caused by the volcanic activity but they occurred nearly simultaneously.
The warning level for the peak was raised to 3, meaning that people should not climb the mountain, the JMA said.
Japan has 110 active volcanoes and monitors 47 of them around the clock.
Reuters contributed to this report
