PURPOSE: To obtain an adaptive digital filter equal to a conventional filter substantially by correcting a coefficient parameter at each sample period so as to decrease the amount of operation for correcting a coefficient parameter.

CONSTITUTION: When a characteristic of a filter 2 whose characteristic is unknown is approximated by an uncyclic type digital filter 1, an output 6 of the adaptive digital filter is zero. If the output 6 is not zero, a coefficient parameter Cj is corrected at each sample with a correction circuit 8 by using the output 6. The output 6 is multiplied with a coefficient (g) at a multiplier. The output of the multiplier is distributed into two lines at each sample by a switch circuit 7. The coefficient parameter is operated alternately at each sample period by the distributed signal. Through the processing method like this, the multiplication circuit required at each sampling time of the adaptive digital filter is decreased to nearly a half.