PURPOSE: To improve the specific filter effect and noise or the like by inputting two signals being digital data inputted and outputted to/from a speaker to an adaptive filter circuit as control information and minimizing the mean square error of input series of the two signals.

CONSTITUTION: An adder circuit 5 takes a difference between an original audio data inputted through a data bus 1 and an audio data inputted through a data bus 12 through an acoustic space and the difference is used as an error signal and given to a data bus 13. A filter coefficient of an adaptive filter circuit 8 is decided so as to minimize the mean square of the error signal. Then weight information of the filter coefficient is latched by a slave filter circuit 1 and the weight of the filter coefficient is copied. Filter processing of the digital audio data using the optimum filter coefficient is applied in the said circuit 1 and its output is converted into an analog data and outputted from speaker with an amplifier.