Authorities of the Nagano prefecture told reporters that four bodies were found on the slopes of the mountain in areas searched Saturday for the first time.
Whether these four victims were among the 16 mountain climbers, who have been missing since Ontake's eruption, has yet not been determined.
The possibility of more people having gone missing has not been dismissed due to the difficulty of keeping count of the number of excursionists who climbed the mountain from different access stations on the day of the eruption.
Ontake, Japan's second largest volcano at 3,067 meters (10,056 feet) and located some 100 km (60 miles) from Nagoya city, began erupting last Saturday while hundreds of mountain climbers were climbing its slopes or had reached its summit.
Some 250 people are known to have left the area or were evacuated, 69 of whom suffered various degrees of injuries, mostly fractures and burns.
Mount Ontake was thought to be inactive until October 1979, when it underwent a series of explosive eruptions, spewing over 200,000 tonnes of ash.