AGC DEVICE

PURPOSE:To attain ease of gain setting by providing a pin diode whose impedance is changed according to the applied voltage thereby changing the equivalent coupling amount of a directional coupler. CONSTITUTION:A part of an input signal is branched by the 1st directional coupler 22, detected by the 1st detector 27, a part of an output signal of an amplifier 24 is branched by the 2nd directional coupler 25, detected by the 2nd detector 28 and each detection output signal is compared by a comparator circuit 29 and the automatic gain is controlled by controlling a variable attenuator 23. The pin diode 30 connected between the 1st directional coupler 22 being a circulator and the variable resistor 23 and whose impedance is changed according to the applied voltage to change the equivalent coupling amoung of the 1st directional coupler 22 is provided at the same time. Since it is enough to set the gain by adjusting the voltage applied to the pin diode 30, the gain is set easily by the variable resistor 32 or the like.