To automatically select a normal load or a short circuit by detecting current for carrying out sampling at each specific time, by outputting time-series data obtained by performing A/D conversion, by comparing a value obtained by carrying out specific operation with a preset setting value for each frequency, and by detecting a failure for protecting.

A high-speed circuit breaker 1 is equipped with a current waveform converter 2 that takes out the differential current wave of a three- phase bus, and a filter 3 that passes a specific frequency band component in the current of the three-phase bus and a differential current being taken out by the current waveform converter 2. In the high-speed circuit breaker 1, an A/D converter 5 and a protection relay 6 are provided, where the A/D converter amplifies an output signal from the filter 3 by an amplifier 4, samples a signal being amplified by the amplifier 4 at each specific time, and outputs time-series data obtained by carrying out A/D conversion, and the protection relay 6 outputs a break command for breaking conduction in a failed place based on the time- series data from the A/D converter 5, thus quickly performing judgment, and improving safety.