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.
ceESgdJoQCcImage source: Wikimedia Commons President-elect Donald Trump celebrated his election victory at the Ultimate Fighting…
Millions of voters across the US chose to return Donald Trump to the White House…
Donald Trump declares victory in the US election as he addresses jubilant supporters in Florida.…
Stocks around the world are rising as Donald Trump appears to be on the cusp…
Donald Trump has won Pennsylvania, North Carolina and Georgia and taken a lead over Kamala…
Quincy Jones, the celebrated musician and producer who worked with Michael Jackson, Frank Sinatra, Ray…