PURPOSE: To automatically adjust the decree of separation by changing the synthesizing ratio of a main channel signal and a sub sound signal in a matrix circuit based on the detected amplitude of a sub channel signal.

CONSTITUTION: At an LPF 2, a high frequency component in the output of a variable gain amplifier 1 is cut, only the low frequency component is passed, and a main channel signal S3 is outputted. That main channel signal S3 is supplied to a matrix 4. At a BPF 3, high and low frequency components in the output of the variable gain amplifier 1 are cut, only the intermediate frequency component is passed, and a sub channel signal S4 is outputted. This sub channel signal S4 is converted into a sub sound signal S5 by a sub sound demodulator 5 and outputted to the matrix 4 later. A sub channel amplitude detector 6 detects the amplitude of the sub channel signal S4, and an error amplifier 7 operates error between that amplitude value and a reference value and outputs the result to the variable gain amplifier 1. As a result, only the amplitude of the main channel signal S3 is changed, and the synthesizing ratio at the matrix 4 is changed.