To stop a vehicle coming toward a tunnel when a signal line is disconnected by the generation of rock fall or the like by photographing an outlet of a tunnel in response to a change of the signal, which is received by a receiver provided in the outlet of the tunnel, and arranging a display unit, which is interlocked with the operation of a camera, at an inlet side of the tunnel.

A signal source and a receiver are arranged in a protecting net 3 through a signal line as a sensor, and a camera 7 to be operated with a change of the signal received by the receiver is provided. In this case, a rock, which is fallen onto the net 3, is photographed by the camera 7 provided on a pole 9. Furthermore, an indicator or a signal is arranged inside of the tunnel or an inlet thereof so as to display the generation of rock fall when rock fall is generated, or a signal 11 is turned to RED so as to alarm a driver of a vehicle. In the case where a part of the signal line is disconnected by the rock, which is fallen onto the net 3, image of a field of the rock fall is transmitted as an emergency road information to a center for controlling the tunnel. An alarm is generated to a vehicle coming toward the tunnel. With this structure, escalation of an accident due to the generation of rock fall is prevented.