"We have confirmed that more than 30 individuals in cardiac arrest have been found near the summit," a Nagano prefecture police spokesman told the AFP news agency.
About 550 soldiers, police officers and firefighters were involved in a large-scale search-and-rescue operation in a bid to save dozens of hikers who were thought to have been stranded on the volcano since it erupted without warning spewing ash, rocks and steam into a sunny autumn weekend busy with tourists and hikers.
About 250 people were initially trapped on the slopes. Most had made their way down by Saturday night but about 40 spent the night near the peak. More than 40 people were injured, several with broken bones. The volcano was still erupting on Sunday, pouring smoke and ash hundreds of metres into the sky. Ash was found on cars as far as 80km away.
Volcanoes erupt periodically in Japan, one of the world's most seismically active nations, but there have been no fatalities since 1991, when 43 people died in a pyroclastic flow, a superheated current of gas and rock, at Mount Unzen in southwestern Japan.