PURPOSE: To prevent malfunction due to chattering and also the malfunction due to chattering whose charging time is longer than the discharge time by discharging a capacitor rapidly for a discharge time.

CONSTITUTION: A comparator 12 is inverted, an NPN transistor(TR) 22 is turned on and a current depending on a resistor 23 for current limit flows through a current mirror circuit comprising PNP TRs 24,25. Thus, a current flows rapidly to a base on an NPN TR 9 to discharge a capacitor 11 rapidly and after the inversion of the comparator 12, the circuit is operated in a sufficiently faster discharge time than the charging time. Then with an external switch 18 turned off, when the comparator 12 is inverted, the NPN TR 22 is turned off and the capacitor 11 is charged/discharged in a charge/discharge time of the capacitor 11 to be set. As a result, malfunction of chattering is prevented and malfunction due to chattering whose charging time is longer than the discharge time is prevented.