PURPOSE: To suppress the fluctuation of an output dynamic range by flowing a current independent of an operating current to a diode provided as a load of a differential amplifier to make an output DC potential of the differential amplifier constant.

CONSTITUTION: An automatic gain control circuit consists of two differential amplifiers 20, 22. Then the current flowing to diodes 30, 32 as loads of the amplifier 20 is set by variable current sources 36, 38 independently of the operating current. Thus, when a control voltage Vc is given, the current of the diodes 30, 32 is increased/decreased, but the output DC potential of the amplifier 20 is kept constant. Since the output of the amplifier 20 is fed to the amplifier 22 having a constant amplification gain, its output DC potential is kept constant. Thus, the output DC potential is kept constant and the fluctuation of the output dynamic range is suppressed.