To simplify construction and to eliminate influence of inductive noise against a signal transmission cable by integrating the signal transmission cable and a cable for surveyance of a conduit run with each other.

An underground transmission system carries out centralized supervision and control of water supply and sewerage conduit runs by transmitting information from a sensor unit 3 provided in the water supply and sewerage pipe lines to a central monitoring room 11 through a cable. In this case, the cable 2 is devised so that a signal transmission cable 8 connected between the sensor unit 3 and a central monitor control device 5 of the central monitoring room 11 and a cable 9 for pipe line surveyance connected to the central monitor control device 5 and laid along the pipe lines are integrated and the cable 9 for conduit run surveyance is changed over to the side of an earth 13 and the side of an electric power source by a change-over switch 12.