PURPOSE: To reduce the time constant of a filter and to increase the AGC response speed by performing the subtraction by a subtractor between the outputs of an AM wave detector having a peak holding function and an AM wave detector having no peak holding function and obtaining a signal containing virtually no AC component in response to the intensity of a carrier wave signal.

CONSTITUTION: A subtractor 12 performs substraction between wave detection signals of an AM wave detector 8 having a peak holding function and an AM wave detector 10 having no peak holding function. Thus the difference between both signals is detected. This difference is turned into a signal which contains virtually no modulated wave component and is proportional to the intensity of a carrier wave signal. The AGC voltage obtained by transmitting the output of the subtractor 12 through an amplifier 14 and a filter 15 is turned into te DC voltage. Furthermore the value of this DC voltage is proportional to the intensity of the carrier wave signal. The AGC signal is applied to a frequency mixing circuit 4 and an intermediate frequency amplifying circuit 6 as the control input. Thus the gain is automatically controlled.