To detect a momentary power failure or a power failure with a residual voltage at high speed by comparing a difference of data at two sampling times spaced via one cycle of an a.c. voltage with a predetermined power failure judgment level.

A voltage of one phase of a three-phase power system detected by a voltage detector 21 is converted to a digital signal by an A/D converter 22, and sampled with a constant electric angle synchronous with the voltage. The data are sequentially stored in a memory 24. A processor 23 reads out data at the present sampling time and at the same sampling time of one earlier cycle and operates a deviation of the data. If no power failure occurs and a voltage waveform is normal, the deviation is zero. If a power failure takes place in one cycle, the deviation shows a certain value. A suitable power failure judgment level is set beforehand. The presence/absence of a power failure is detected by comparing the deviation with the level.