PURPOSE: To change the time constant without using a variable resistor by providing q control voltage generating circuit outputting a voltage corresponding to a clock frequency and controlling the current of the constant current source by said voltage.

CONSTITUTION: Since the current of a switched capacitor circuit is changed in response to a clock frequency, the change is detected and the voltage CNT is generated by an operational amplifier 30 and a capacitor C21. The voltage CNT controls the current of the constant current source of a differential amplifier section 11A of the filter section. Thus, the current flowing to the capacitor C11 is set by using a clock and the inverse of clock .