PURPOSE: To attain ease of filter design by replacing as it is a characteristic formula of an analog filter to a digital circuit to simplify the relation between a coefficient of a multiplier and the filter characteristic.

CONSTITUTION: The characteristic formula of an analog BIQUAD filter is into a digital circuit and outputs of HPF/BPF/LPF are obtained respectively as outputs of adders 9∼11. In changing a coefficient 1/τ of multipliers 12, 13, the cut-off frequency is changed, and when the coefficient F of a multiplier 14 is changed, the Q of the filter circuit is changed. As a result, in changing the filter characteristic, the number of coefficients to be changed is decreased.