To calculate the coefficient of an FIR filter and to compensate. the amplitude and group delay distortions by designating the amplitude and group delay characteristics as the compensation characteristics at a designated point of frequency based on the characteristic that is actually measured and applying the inverse FFT conversion to the amplitude and group delay characteristics.

The signal that is outputted through a filter which compensates the signal distortions and via a compensating object device is extracted, and the output characteristic of the device is measured (11) based on the extracted signal. Then the amplitude and group delay characteristics are designated (12) so as to eliminate the difference between the measured output characteristic of the device and the reference characteristic. The linear compensation is applied to the amplitude and group delay characteristics (13) and the frequency characteristic is measured (14). The inverse FFT data is calculated (15) from the calculated frequency characteristic and the result of this calculation is defined as a compensation coefficient (16). Then the compensation coefficient is inputted to an FIR filter (17) and the output signal is compensated. If the compensated output signal is not identical to the desired one, an output signal can be extracted again (18).