To specify a collapse position of a tunnel from a change in electric field intensity of a radio wave by arranging a radio wave-transmitting means and a radio wave-receiving means in respectively difference positions with every prescribed distance in the inside of a tunnel.

PHS base stations 111 to 11n are arranged at almost equal intervals in a tunnel 101. A collapse detecting terminal 13m is installed on a wall opposed to the base stations in the center of the two base stations in place where left and right zones overlap each other. The respective base stations 11n are connected to a center yard exchange 141 and a computer 142 through an ISDN 15m. The yard exchange 141 has a movement control mechanism 143, and informs the computer 142 of the collapse detecting terminal 13m and existent zone information. When a radio wave passing hole is partially bored by collapse, electric field intensity is reduced, and when it is completely blocked up, the electric field intensity becomes zero and a collapse position can be known. An image pickup terminal automatically moves to a collapse site, and takes a proper measure by displaying it as a monitor image by transmitting image data and taking a photograph.