To quickly open a bus-connected circuit breaker by shortening the detecting time of the service interrupt fault of a commercial system by detecting the fault from the voltage and period of the generator output of a private power generation facility.

A waveform shaping circuit 244 shapes the waveform of an AC voltage into a pulse waveform (rectangular waveform), and a counter 245 measures the half-period time interval of the pulse waveform at high level (or low level). When the counter 245 counts up during the half-period of the waveform, the counter outputs a frequency drop signal SF, and an AND circuit 248 outputs a cut-off command under the AND condition of the signals SF and a voltage drop signal SV. When each phase of a three-phase alternating current and their inverted phases are shaped into pulse waveforms and the half-periods of the pulse waveforms are measured by means of separate counters in this constitution, a frequency drop can be detected within 2/3 cycle in maximum, and the cutting-off of a non-utility generation facility from a commercial system can be executed at high speed.