An AGC circuit according to the present invention comprises: a gain controller, having a variable gain, for amplifying an input signal at a predetermined gain and generating an output signal, and for varying the gain; a comparison circuit for comparing an amplitude of the input signal or of the output signal with a predetermined reference level, and for generating a control signal that goes to a first level when the amplitude is smaller than the reference level, and that goes to a second level when the amplitude is greater than the reference level; and a duty ratio detector for supplying the control signal generated by the comparison circuit, and for generating control code in accordance with a duty ratio of the control signal, wherein the gain of the gain controller is varied by using the control code, and so adjusted that the duty ratio of the control signal is maintained to be a predetermined value. When the amplitude of the input signal or the output signal is increased, a period during which the amplitude exceeds the reference level is also extended. By utilizing this phenomenon, a gain is varied so as to maintain a control signal duty ratio of 50%, for example, so that the amplitude of the output signal is amplified to a desired value, which is maintained.