PURPOSE: To prevent the signal of a loop back test code from occupying a transmission line by eliminating the setting of the loop back test code for respective devices and detecting plural fault places on the transmission line.

CONSTITUTION: A loop back test signal is obtained by overlapping direct current from a transmission line-side constant current source 101 and a reception line- side constant current source 103. Then, the transmission line overlapping and transmitting direct current is switched by using a transmission line-side feedback overlap signal transfer switch circuit 118, a reception line-side feedback overlap signal transfer switch circuit 121, a transmission line-side overlap signal transmission line changeover switch circuit 123 and a reception line-side overlap signal transmission line changeover switch circuit 124. Thus, the plural interrupted places occurred on the transmission lines in the same direction can be detected.