PURPOSE: To vary continuously the gain of an amplifier in a wide range of frequency with a simple circuit constitution by controlling the gain of the amplifier in response to the received light volume of a photodetector connected to a feedback circuit of the amplifier.

CONSTITUTION: A control input is applied to a control input terminal 13 to control a transistor TR11, and therefore the quantity of current flowing to an LED9b of a photocoupler 9 is varied to control the emitted light volume. Thus the resistance value of a CdS9a of the light emission side is reduced in adverse proportion to the emitted light volume. Thus the negative feedback quantity is decided by the synthetic resistance of a feedback resistance 5 and the resistance of the CdS9a. Then the gain is decided in relation to a resistance 2.