PURPOSE: To facilitate detection of commercial power interruption on the side customer by a simple system, and to parallel off the customer side from the commercial power supply system surely and automatically.

CONSTITUTION: The commercial power supply system 15 of a substation is provided with a signal transmitting circuit 30, for normally injecting high frequency signals S1 having higher frequency than the commercial frequency S0 through commercial power supply, and a signal receiving circuit 31 is provided on the side of a load 6, which is linked up with the commercial power supply system 15 and has a switch 12 for linking, and this signal receiving circuit 31 is connected electrically to the commercial power supply system 15 and sends out a release signal S2 which releases the switch 12 for linking only when it does not receive the high frequency signal S1.