Thirty hikers have been found lifeless near the peak of Mount Ontake, after Saturday’s sudden volcanic eruption.
The hikers were not breathing and their hearts had stopped, reports said. Final confirmation of death in Japan always comes via a medical examination.
About 250 people were trapped on the slopes of the popular beauty spot, but most have got down safely.
The volcano, about 200km (125 miles) west of Tokyo, erupted without warning on Saturday, spewing ash and rocks.
The eruption forced many of those on the mountain to make emergency descents through clouds of volcanic ash and falling rocks.
Almost 50 people were thought to have stayed on the mountain overnight, reports said.
As the search effort intensified earlier on Sunday, officials said they were searching for 30 people feared missing or buried by ash.
Confirmation soon came that a similar number of people had been found unresponsive on the mountain.
“We have confirmed that more than 30 individuals in cardiac arrest have been found near the summit,” a Nagano police spokesman told the AFP news agency.
Military helicopters plucked seven people off the mountainside earlier on Sunday, according to reports, and workers on foot were also helping others make their way down.
Japan is one of the world’s most seismically active nations but there have been no fatalities from volcanic eruptions since 1991, when 43 people died at Mount Unzen in the south-west.
The BBC’s Rupert Wingfield-Hayes, in Tokyo, says it’s not clear why there was no warning of Saturday’s eruption.
Japan monitors its volcanoes closely and any that show signs of activity are immediately closed to hikers – but this time that did not happen.
The sudden eruption on Saturday was described as “like thunder” by one woman who runs a lodge near the summit.
Heavy, toxic volcanic ash up to 20cm (8in) thick covered much of the mountain, reports said.
“All of a sudden ash piled up so quickly that we couldn’t even open the door,” Shuichi Mukai, who worked in a mountain lodge just below the peak, told Reuters.
“We were really packed in here, maybe 150 people. There were some children crying, but most people were calm. We waited there in hard hats until they told us it was safe to come down.”
Ordinarily Mount Ontake is a popular place to see autumn foliage.
Its peak is 3,067m (10,120ft) high and the mountain is a popular hiking route, dotted with lodges, cabins and well-marked trails.