To make it possible to update I/O characteristic even if calculation does not end within a fixed period by asynchronizing with sampling a point of time for updating an input-output characteristic of a digital signal processing system and determining it according to a scale of the digital signal processing system and an arithmetic capacity of an adaptive processor.

Creation of muffling wave signals y1(t) and y2(t) done by a muffling wave signal creation part 12 are executed at each sampling cycle. An adaptive processing part 13 updates filter factors by using a reference signal x(t) and an error signal at the time of starting updating the filter factor. Updating of the adaptive filter factor performed by the adaptive processing part 13 is executed asynchronously with sampling, and once the filter factor has been updated, the next updating is executed in no time. Thus, since the filter factors are asynchronously updated with the sampling clock, time limitation is eliminated for updating the filter factors. As a result, muffling becomes possible even if updating of filter factors does not end in one sampling cycle.