PURPOSE:To realize the approximation of the desired frequency (amplitude) characteristics on a nonlinear frequency scale like a mel scale, etc., by converting the delay element of a nonangular feedback type filter into a full band pass filter. CONSTITUTION:It is supposed that the transmission function H(z) of a digital filter to be attained is obtained by the polynomials (1) and (2) of the transmission function HA(z) of an appropriate full band pass filter. The frequency characteristics HA(e<j> ) is shown by the formulae (3) and (4), where OMEGA=omegaDELTAt, omega: angular frequency, DELTAt: unit delay time of digital filter. Accordingly the frequency H(e<j> ) of the desired filter is shown by the formula (5). The coefficient gm is selected so that the approximation is possible to the desired frequency characteristics G(OMEGA') given with a nonlinear frequency OMEGA' with the value of the formula (5).