PURPOSE: To magnify the dynamic range of a video signal with respect to an illumination light by providing a variable gain amplifier whose gain is selected in a dimmer control loop, increasing the gain of the variable gain amplifier when the output level of a solid-state image pickup element is reduced or the like and correcting the level decrease in the video signal due to the deterioration in the luminous quantity.

CONSTITUTION: A variable gain amplifier 24 amplifying a video signal is provided in a dimmer control loop controlling a diaphragm 12. A prescribed gain used to increase a video signal is provided on a video signal inputted to the variable gain amplifier 24 in its operation. A gain control signal generating circuit 56 implements the gain switching control of the variable gain amplifier 24 and controls the dynamic range of the video signal with respect to the illumination light to be expanded based on each detection output of level detection circuits 51, 52 detecting the level of the video signal at the input terminal and the output terminal of the variable gain amplifier 24 respectively and an diaphragm fully open detection circuit 53 detecting the fully opening state of the diaphragm 12.