PURPOSE: To obtain a higher harmonic filter which operates accurately to an integer-fold frequency compared with the basic frequency and has an appropriately high Q with a high speed of response, by supplying a frequency obtained by multiplying the basic frequency as the clock of the N-path filter.

CONSTITUTION: For the input signal, the basic frequency is obtained through the basic frequency recognizing circuit 1 and then multiplied by the multiplier 2 to be sent to the clock control circuit 3. Thus the ratio of division or the duty ratio of the pulse is varied to control the N-path filter group 4. For the filter group 4, the central frequency of the pass band depends on the frequency of the clock pulse. Then the clock pulse of double the higher harmonic component to be detected, i.e., the basic frequency is set to f0. Thus the clock pulses of 4f0 and 6f0 are supplied to the switches S1, S1', S2, S2'...Sn, Sn' for the detection of the 2nd and 3rd higher harmonic components respectively.