PURPOSE: To prevent a comparator from being not in operation and malfunctioning owing to a noise, by smoothing an input signal to the comparator and then obtaining a reference voltage through voltage division.

CONSTITUTION: Part of an input signal to a comparator A is inputted to an integration circuit B through a diode D for stopping a reverse current. The integration circuit B generates a specified voltage in a proper period in response to even an impulsive input signal. The output voltage of the integration circuit B is divided through resistances R1 and R2 to obtain the reference voltage of the comparator A. The reference voltage of the comparator A is determined according to the voltage of the input signal, so the comparator operates by following up even variation in input signal level, and a little influence of a noise is exerted.