PURPOSE: To easily specify the quality degradation section by applying the same signal from a transmission terminal station to an in-use line to be tested and a stand-by line and sending the output signal of a stand-by reception and demodulation part out of each repeater station requentially through the in-use line to be tested.

CONSTITUTION: When a reception terminal station 15 detects the error rate of data from an in-use line (M1 and M2), e.g. M1 decreasing below a prescribed value, a signal 1 is applied to the stand-by line (PROT) as well. Here, recepter station 13 sends the signal 1 passing through the PROT, to the reception terminal station 15 through the M1 and the reception terminal station 15 compares the error rate of the signal passing entirely through the M1 with the error rate of the signal 1 passing through the POT only between the transmission station 12 and repeater station 13 to judge that the quality degradation section is not the M1 between the transmission terminal station 12 and repeater 13 when they nearly coincide with each other, and then performs the operation, at every repeater. Consequently, the quality degradation section can easily be specified.