To obtain control characteristics that the brightness of light emission is defined as a prescribed fixed value when object illuminance is equal to or less than a first threshold and the brightness of the light emission is linearly reduced according to the object illuminance when the object illuminance is higher than the threshold by simple circuit structure in a monitoring camera which performs the automatic control of the brightness of the light emission of an illuminator by using a video signal.

When the object illuminance is dark and an output voltage of an invertible amplifier circuit 12 is equal to or more than a forward voltage of a diode D of a limiter circuit 13, an input voltage of a constant current circuit 14 is restricted to the forward voltage of the diode D. When the object illuminance is raised and the output voltage of the invertible amplifier circuit 12 becomes less than the forward voltage of the diode D of the limiter circuit 13, the output voltage of the invertible amplifier circuit 12 is inputted in the constant current circuit 14 as it is.