The bodies of 30 hikers were found near the peak of Mount Ontake, about 200 km west of Tokyo, which erupted Saturday without warning, BBC reported.
About 250 people were trapped on the slopes and in a hotel near the top of the popular beauty spot, according to Xinhua.
The official announcement of deaths in Japan always comes after a medical examination.
Although rescue work could not be started Saturday due to the potential of a secondary disaster, search efforts intensified Sunday. Authorities said that besides the 30 "unresponsive" bodies found, 30 people were feared buried under volcanic ash.
Japan is one of the world's most seismically active countries. In 1991, at least 43 people died when Mount Unzen erupted in the country's south-west region.