Witnesses heard a “thunder-like” boom before the eruption at the 3,067m (10,120ft) peak, situated between Nagano and Gifu prefectures.
PM Shinzo Abe says the army will help rescue some 150 climbers sheltering in lodges near the summit.
Officials have warned residents within a 4km radius of the risk of debris.
Mount Ontake last erupted in 2007.
Some climbers have managed to descend.
Two of those injured have fractured bones caused by flying rocks, NHK said.
Three people are missing, believed buried under volcanic ash, it said. A fourth was rescued but remains unconscious, the broadcaster added, citing police.
Kiso Prefectural Hospital, near the mountain, said it had dispatched a medical emergency team.
“We expect a lot of injured people so we are now getting ready for their arrival,” said an official at the hospital.
ceESgdJoQCcThree Indian nationals have been arrested and charged over the killing of Sikh separatist leader…
President Joe Biden has urged pro-Palestinian protesters on university campuses to uphold the rule of…
Blue Ivy Carter has joined the voice cast of The Lion King prequel Mufasa: The…
At least five people, including a four-month-old baby, have been killed after dozens of tornadoes…
Harvey Weinstein has been hospitalized just days after his 2020 rape conviction in New York…
Hamas has published a video showing the first proof of life of US and Israeli…