PURPOSE: To vary the center frequency of the path band and the pass band width with simple constitution by constituting a band-pass filter by an interleaving circuit, an interpolation circuit, an LPF and a multiplier in combinations.

CONSTITUTION: In multiplying a cosine wave with a digital signal by a multiplier 1 and a sine wave by a multiplier 9, the signal pass band frequency is shifted to the base band and a high frequency region is cut off by the LPF. In multiplying a cosine wave by a multiplier 8 and a sinuosity wave by a multiplier 17 with the resulting signal, the input signal frequency is shifted to the original frequency region. In selecting the interleaving at a rate of 1/l, the pass band width is narrowed by 1/l as number of times of feedback is increased once, the calculation quantity is less than n-time that per one feedback even with n-time feedback thereby reducing the calculation time. Thus, the pass band center frequency and the pass band width are changed through simple constitution.