A police spokesman, Yoshihiro Fukutani, said police were still trying to learn the exact situation inside the Sasago Tunnel, about 50 miles (80 kilometers) outside the city, after parts of a wall inside collapsed early Sunday morning.
Television footage showed police vehicles, fire trucks and ambulances massed outside the tunnel's entrance.
A man who said he saw the collapse and alerted authorities to the emergency managed to escape after he was ordered to flee.
The roof and windows of another vehicle parked on the roadside outside the tunnel were crushed; the injured occupants reportedly taken to a hospital.
