PURPOSE: To decrease circuit scale in the case of making the digital low-pass filter of associated Chebyshev characteristics and to perform Chebyshev approximation to the frequency characteristics of zero-order hold in a passing band.

CONSTITUTION: The summarized characteristics are provided by a pre-filter of which filter coefficient is an integer. Next, the characteristics of this pre-filter are equalized by an equalizing filter so that the total characteristics can satisfy by desired characteristics. The integer coefficient of this pre-filter is set so that zero points can be arranged on a unit circle in a block area, and the zero points are installed so as to be flat in the passing On the other hand, the frequency characteristics of the amplitude in the zero-order hold of this equalizing filter are corrected by performing the Chebyshev approximation in the passing band.