PURPOSE: To discriminate the fault of a subscriber's line easily through the AND operation between a return signal and a photo coupler detection signal which indicates whether or not an automatic return signal has flowed.

CONSTITUTION: Terminating set 14 detects an automatic return operation signal from intra-office device 3 of the system which makes base band transmission through subscriber's lines 12 and 13, and device 14 makes the return connections of transmitting and receiving subscribers. In device 3, photo coupler 23 detects an automatic return operation signal, a return signal is generated and then sent to subscriber's line 12 return-connected from transmission part 5 of device 3 at device 14, the return signal received by return signal reception part 7 of device 3 and the signal detected by photo coupler 23 are AND-ed 23. Consequently, the fault of a subscriber's line can be discriminated easily