PURPOSE: To prevent incidence of step-difference noises with a simple constitution by demodulating two different sound signals which are frequency-modulated by plural frequencies and supplying their difference frequency signal to a frequency converting circuit, and then, converting and reproducing one side signal to the other side signal.

CONSTITUTION: Sound output signals of limiter circuits 421 and 423 are switched to outputs of limiter circuits 422 and 424 outputted through multiplying circuits 70L and 70R by switching circuits 72L and 72R and supplied to demodulators 43L and 43R through BPFs 73L and 73R. On the other hand, a signal from the BPF 73L is supplied to a phase comparison circuit 76 through a frequency dividing circuit 75 of 1/N1 and the output of the circuit 76 is supplied to a voltage controlling oscillator 78 through an LPF 77. The output of the oscillator 78 is supplied to multipiers 70L and 70R and, at the same time, to the comparator 76 through a frequency dividing circuit 79 of 1/N2. If a phase error occurs with the output of the oscillator 78 when sound FM waves ALF1 and ALF2 are reproduced, the oscillator 78 is controlled so as to minimize the error and incidence of step- difference noises after demodulation is prevented.