To make connection in parallel to a power supply system supplied from an electric enterprise company with a simple, accurate configuration by judging whether an independent operation or not, by detecting a difference in changes of reactive power component between a system linkage operation and independent operation on a power receiving circuit in a power supply system.

An independent operation judging function 31b sends out a signal for enlarging a fluctuating amplitude of a delay current outputted from a fluctuating source 30 if there is no differential signal at the timing of the input of differential signal. Thus, the fluctuating amplitude of a delay current outputted from the fluctuating source 30 is enlarged. Even though the fluctuating amplitude of a delay current outputted from the fluctuating source 30 becomes larger, if a differential signal is not inputted to the independent operation judging function 31b at the output timing, the independent operation judging function 31b judges a start of independent operation, by which the opening function 4a is operated and a power receiving breaker 4 is opened. Thus, the power output of non-utility AC power generator installed by a power consumer is not reversely flowed to the power supply system.