PURPOSE: To improve the stability of a frequency characteristic against an environmental change and a secular change.

CONSTITUTION: The filter is formed by externally mounting required resistors R3-R6 and a capacitor C3 to a filter forming IC 21 having a state variable type filter circuit section formed by connecting a summing amplifier circuit and a switched capacitor filter circuit in cascade. The frequency characteristic is decided by a clock pulse executing switching control of the switched capacitor filter circuit. The clock pulse is generated by frequency-dividing a reference pulse generated by a crystal oscillator 3 at a preset frequency division ratio at a frequency divider 4 to improve the stability of a frequency characteristic against an environmental change and a secular change.