DIGITAL FILTER

PURPOSE:To improve the stability of an IIR filter by providing an output setting means to the IIR filter in a digital filter comprising the cascade connection constitution of a FIR filter and the IIR filter. CONSTITUTION:The FIR filter 109 having plural zeros on a unit circle of the Z plane and the IIR filter 114 having a pole on a coordinate Z=1 are connected in cascade and an output setting means 111 is provided to the output side of the IIR filter 114. The output setting means 11 is set once at a prescribed point of time, e.g., at application of power supply. Thus, the output of the IIR filter set to an undesired value is prevented, then malfunction is prevented. Further, the zero of the FIR filter and the pole of the IIR filter are made completely coincident on the unit circle.