PURPOSE: To reduce cost by simplifying the circuit by unnecessitating an electrolytic capacitor for bias input stabilization by inputting a video signal to an automatic gain control (AGC) circuit in DC direct connection.

CONSTITUTION: The input video signal is selected by changing over a switch SW, attenuated just for a prescribed amount by an attenuator ATT 1 or 2, passed through a buffer circuit 10, level-shifted by a resistor R1 for level shift and inputted through a buffer circuit 11 to an AGC circuit 3. This AGC output signal is passed through a buffer circuit 12, level-shifted by a level shift circuit 13 and supplied through a buffer circuit 14 to a feedback clamp circuit 41. In this case, the AGC output signal is compared with a reference voltage E2 by a differential amplifier 40, and that output is integrated by a capacitor C3 for integration and fed back to the circuit 11. Then, the voltage E2 is selected so that the DC level of the input video signal of the circuit 3 can be equal to a reference bias signal voltage VB. Thus, the electrolytic capacitor for AGC circuit bias stabilization is unnecessitated, and the linearity of the circuit 3 is not damaged as well.