PURPOSE: To obtain a highly accurate high speed commercial power supply monitoring circuit for uninterruptible power supply in which erroneous function and erroneous decision are eliminated both when the voltage of commercial power supply has a waveform approximate to a pure sine wave and when it has a peak-cut waveform.

CONSTITUTION: A sin table 6 and a distorted wave table 7 prestored in a memory element are selected by a selected switch 11 while being switched every half cycle based on the conditions of a zero cross detector 3, a waveform monitor/distortion decision unit 8 and an effective value monitor/allowable value decision unit 10. Comparison voltages V4, V5 for monitoring a commercial power supply voltage V1 having peak-cut waveform (solid line) while holding between is shown by solid line and comparison voltages V4, V5 for monitoring the commercial power supply voltage V1 having a waveform approximate to pure sine wave (dot line) is shown by dot line.