PURPOSE: To surely supervise the normality of a standby line by sending the 1st and 2nd pseudo random signals having the same pattern and a large bit deviation to the standby line as a test signal from a pseudo random signal of a transmission station.

CONSTITUTION: The pseudo random signal generator 3 of the transmission station 1 outputs outputs the 1st pseudo random signal (a) and the 2nd pseudo random signal (b) having the same pattern as that of the signal (a) and deviated by a prescribed bit number, gives them to a transmission section 10 of the standby line via a switching circuit 11 and sends them to the standby line as the test signal. A reception station 2 supplies the test signal received by a reception section 12 to a supervisory section 4 via a switching circuit 13. Th supervisory section 4 compensates the bit deviation of the received signals a, b to make them coincident, compares the signals a, b to supervise the normality of the standby line.