PURPOSE: To obtain a detection circuit for service interruption of an uninterruptible power supply device which can detect power interruption and reception of power speedily and accurately.

CONSTITUTION: The title item consists of an A/D converter 85 for converting the corresponding value of an AC input voltage to a digital value and a microcomputer 86 for storing the digital value corresponding to the waveform on service interruption and reception of power, inputs a digital value from the A/D converter 85 to the microcomputer 86, compares the digital value with that stored at the microcomputer 86 by operation, and then transmits the detection signal of service interruption and reception of power. An instantaneous value of AC input voltage is converted to a digital value for operation and at the same time digital values corresponding to the waveform on service interruption and reception of power are calculated and compared, thus detecting service interruption and reception of power speedily and accurately.