At least two cars remained trapped Sunday after the tunnel collapse, according to the East Yamanashi Fire Department. Fire officials said the section of concrete that fell was about 50 to 60 meters long and about 20 centimeters (8 inches) thick.
Early reports indicated that the collapse killed three people and seven others were unaccounted for.
The Sasago tunnel is on the Chuo Expressway. The cave-in occurred on the Tokyo-bound lanes, Otsuki police said.
The road was temporarily closed, and authorities were working to rescue victims, police said.
Japanese public broadcaster NHK aired images from the tunnel, showing smoke rising and a blue car with its side smashed in. Emergency vehicles were on scene.
A journalist from NHK was driving through the tunnel at the time of the collapse. He told the broadcaster his car was damaged, but he was able to make it out unhurt.