PURPOSE: To provide an independent operation detecting method which detects that the parallel-off of an inverter for system interconnection of a distributed power source in interconnection with a power system occurs and independent operation starts.

CONSTITUTION: A reactive power varying command of a constant period is issued from a variation signal generator 11 to supervise the frequency of a power receiving point, thus causing a timer circuit 16 to start in case that a frequency rise supervisory circuit 13 or frequency drop supervisory circuit 14 should operate, and an independent operation signal is outputted in case that an upper variation limit supervisory circuit 18 or upper variation limit supervisory circuit 19 should operate after the measurement of a specified time, so that independent operation can be detected.