PURPOSE: To faithfully reproduce a surround signal by providing a sum signal generating circuit and a difference signal generating circuit which take the sum and the difference between two encoded signals respectively and delaying the difference signal from the difference signal generating circuit to obtain a delay signal and adding the delay signal and the sum signal from the sum signal generating circuit to obtain a monaural signal.

CONSTITUTION: Signals SL and SR of two channels are inputted to a sum signal generating circuit 15 and a difference signal generating circuit 16 through input terminals 11 and 12. The difference signal generating circuit 16 outputs the difference between signals SL and SR, and the difference signal from the circuit 16 is delayed by a delay circuit 17. The sum signal from the sum signal generating circuit 15 and the difference signal delayed by the delay circuit 17 are added by an adder 13, and the result is led out of an output terminal 14 as the monaural signal. The monaural output obtained in this manner includes all channel components where the surround signal is added to signals from each orientation, and the faithful monaural signal is reproduced.