PURPOSE: To prevent the generation of sag, by providing plural comparison detecting circuits having different time constants and switching the circuits with an external control pulse for selecting a comparison detecting circuit having a long time constant for a specific period only.

CONSTITUTION: An input signal Vi is inputted to a front stage amplifier circuit 11, an amplified output is inputted to a video detecting circuit 12 for detection and a detected output Vo is outputted to a video amplifier stage of the rear stage. The detected output Vo is inputted to the comparison detecting circuits 13-1 and 13-2. The output of the comparison detecting circuits 13-1, 13-2 is connected to terminals 15-1, 15-2 in the switching circuit 15, and the output of the two comparison detecting circuits is switched with a control pulse Vc for a limited period and the selected output is applied to the front stage amplifier circuit 11 as an AGC voltage output. The control pulse Vc is obtained so that the synchronizing signal when a sag takes place is delayed and the pulse is given in one horizontal period.