To attain integration by constituting an AGC circuit without the use of analog circuits.

This AGC circuit is provided with a gain control circuit 50, comparator circuits 60 and 62 for comparing the amplitude of an input or output signal with a prescribed level and for generating a control signal which is turned into a first level, when it is smaller than the comparison level, and turned into a second level, when it is large than the comparison level, and duty rate detecting circuit 80 for generating a control code corresponding to the duty rate of the control signal, and gain is controlled so that the duty ratio of the control signal can be maintained to be a prescribed value. Thus, the gain is controlled such that the duty ratio of the control signal can be maintained to be, for example, 50% by utilizing the fact that when the amplitude of the input or output signal is mode large, a period when it is beyond the comparison level is also made longer, so that the amplitude is amplified and maintained to a desired value.