Filtering containing a delay by a specific time is performed on one of the pair of input signals are input from microphones L, R. After the filtering, a pair of input signals InL and InR are alternately interchanged for each sampling by an interchanging circuit 2 to generate a pair of interchanged signals InA and InB. The one interchanged signal InB is multiplied by a coefficient m by an coefficient updating circuit 3 to generate an error signal of the interchanged signals InA and InB. The recurrence formula of the coefficient m containing the error signal is calculated to update the coefficient m for each sampling. The pair of input signals InL and InR are multiplied by the sequentially updated coefficient m and are output.