PURPOSE: To obtain a BPF with a simple constitution in a signal system with less insertion error by changing a center frequency in many ways by one BPF.

CONSTITUTION: A signal from a signal source 16 is applied to a switched capacitor filter 14. The center frequency fOA of the filter 14 is decided as fOA=Kfck, where fck is a clock frequency and K is a constant. Further, the frequency fck is multiplied by 1/2f0, 1/4f0∼1/2n f0 by an oscillator 11 comprising a crystal oscillator or a multivibrator or the like and a frequency divider 12 frequency-dividing its oscillating frequency f0 and the multiplied result is selected by a coded signal from a control terminal of a multivibrator 13 in accordance with the truth table. This signal changes the center frequency of the BPF as a clock frequency of the filter 14.