PURPOSE: To eliminate a time change to be caused for a period from the interruption of the power voltage to a reset signal is outputted by using a Zener diode which controls the output voltage of an integrating circuit down to a prescribed level or less.

CONSTITUTION: The averaged voltage applied to a comparison means 31 is limited down to a prescribed level or less by the Zener diodes 4 - 6. Therefore, the output voltage of an integrating circuit 9 is fixed at a prescribed level in a power supply state despite some difference of the level of the power voltage and the working state of an equipment which are used at the relevant time point. Then the output voltage of the circuit 9 is always reduced along a prescribed reduction curve when the power voltage is interrupted. As a result, no time change is caused for a period when the output voltage of the circuit 9 reaches the reference voltage level of the means 31 from the start of reduction of the output voltage.