PURPOSE: To attain a supervisory system constituting easily and economically supervisory lines by constituting relays installed to terminal station devices and repeaters and their contacts into a closed circuit at a failure so as to measure a DC loop resistance.

CONSTITUTION: Make contacts of relay contacts are constituted in series and brake contacts are constituted in parallel. For example, if a repeater 2 is failed, a realy 5 is activated, its contact 5" is closed and its contact 5''' is opened. Further, since no signal is transmitted to a repeater 3 when the repeater 2 is failed, a relay 6 is also activated. Then, a loop through a relay contact 4''' is constituted when the loop is viewed from a measuring device 10, and the discrimination of location is attained by measuring this loop resistance. In the supervisory system discriminating the failed part by the measurement of loop resistance, since the system is constituted by a couple of supervisory lines, the economy and ease of measurement are attained.