PURPOSE: To prevent an unstable operation from occurring due to noises of a CR oscillation circuit at a low frequency area by connecting a filter capacitor between the differential input of comparators, and connecting it to a capacitor for oscillation via filtering resistor.

CONSTITUTION: A filter part 8 is connected to the input of a window comparator Q5. A filter capacitor Q11 is provided between the reference potential 5 of a compartor Q5 and input. A filtering resistor Q12 is connected between a connection point 6 of a parallel connection element with the drain of a P- transistor Q1 and an input contact 7 of the capacitor Q11 with the comparator Q5. The erroneous inversion of the comparator Q5 due to a high frequency noise can be prevented from occurring by connecting the filtering capacitor Q11 between the reference potential 5 of the comparator Q5 and the input. There are a noise on a power source line and the one from an outfitted CR as the high frequency noise, however, the erroneous inversion can be prevented from occurring since potential difference between both positive/negative input terminals of the comparator Q5 can be kept by the capacitor Q11 even in either case.