PURPOSE: To stabilize the output of a demodulating circuit by turning off the transistor (TR) of a differential amplifier when changing stereophonic broadcast reception over to monaural reception, and by adjusting the gain of a TR connected to it.

CONSTITUTION: The collectors of TRs Q1 and Q2 in one differential TR couple of a differential amplifier are connected in common, and a resistance R3 is connected between the bases of the TRs Q1 and Q2. The collectors of TRs Q3 and Q4 facing to the TRs Q1 and Q2 are connected in common, and a resistance R8 is connected between the bases of the TRs Q3 and Q4. When the intensity of an input signal from a switching circuit 6 decreases below prescribed one, TRs Q5 and Q6 turn on, and the TRs Q2 and Q3 turn off, stopping the supply of a 38kHz signal from a waveform shaping circuit 5 to a chopper circuit B. Consequently, stereophonic reception is changed over to monaural reception, and the deterioration of the SN ratio is prevented.