PURPOSE: To compensate a signal of a power distribution line effectively by improving S/N of signal transmisson, by making it possible to fine an optimum installation position for a repeater through simple constitution.

CONSTITUTION: Phase detection circuit 61 of repeater 6 detects the phase of a commerical frequency applied to power distribution lines 2 and 3 being 0 and π/2; when the phase is 0, 1st gate signal generating circuit 62a generates a rectanqular gate signal and when π/2, 2nd gate signal generating circuit 62b generates a gate signal. On detecting injection signal T1 of a high frequency superposed upon the commerical frequency from a master station or injection signal T2 from a 1st subordinate station, signal detection circuit 63 generates its output to activate AND gate circuit 64a; when signal T1 is detected, its gate operation is brought to effect at a phase of zero to let 1st display circuit 65a come on and when signal T2, 2nd display circuit 65b is made to come on at π/2 in phase. Further, when circuits 65a and 65b both go off, an attenuation value is checked to detect sizes of signals T1 and T2 received by repeater 6, thereby finding its optimum installation position.