To exactly detect a signal level by performing level adjustment by executing A/D conversion of a signal before the signal level is changed by a gain variable amplifier.

A signal outputted from a gain variable amplifier 201 is converted to a digital signal by a first A/D converting circuit 100, inputted to a level detecting and comparing circuit 203 and outputted from an output part 205 to the outside. A video signal inputted to a signal input part 200 is inputted to a second A/D converting circuit 101 as well, converted to a digital signal by the second A/D converting circuit 101 and inputted to the level detecting and comparing circuit 203. In the level detecting and comparing circuit 203, not only the signal level outputted from the A/D converting circuit 100 but also a signal before inputting to the gain variable amplifier 201 are converted to digital signals by the second A/D converting circuit 101, a light source irradiating an object is judged corresponding to the output level, and the variable range of a gain is limited.