PURPOSE: To obtain a circuit for detecting the low voltage side of a power supply voltage drop detecting level before detecting its high voltage side in a computer power supply voltage monitoring circuit for supplying power especially by battery driving.

CONSTITUTION: Two power supply voltage levels are set up by both comparators 1, 2 and power supply alarm detection level signals 7, 8 are outputted. The minimum voltage value of power supply variation which is conventionally detected after the detection of an alarm detection level can be detected before the detection, and when the alarm detection is binarized, battery voltage in the continuous printing of a printer e.g. can be monitored. Consequently the drop of the battery voltage in the continuous printing can surely be detected and power supply forced OFF due to H/W can be evaded.