PURPOSE: To obstruct a level variation and a phase deviation in a filter output signal by providing a band-pass filter constituted of a switched capacitor circuit in accordance with a prescribed input signal, and a clock signal generating means for driving having a frequency of an integer multiple of a frequency of its output signal.

CONSTITUTION: A narrow band-pass filter is constituted of a band-pass filter 1 constituted of a switched capacitor circuit, a phase difference detecting circuit 2, a low-pass filter 3, a voltage controlled oscillator 4, and a frequency divider 5. By such a constitution, one phase locking system is formed by the circuit 2, the filter 3, the oscillator 4 and the frequency divider 5, and in a stationary state, a frequency of a clock signal for driving sent to the filter 1 from the oscillator 4 is set to N times of a frequency of an output signal of the filter 1. On the other hand, the center frequency of the filter 1 is designed to 1/N of the frequency of the clock signal, therefore, a frequency of an object tone signal in a signal inputted to the filter 1 through a terminal 101 always coincides with the center frequency of the filter 1, and a normal output is generated to a terminal 102.