An electronic endoscope including a CCD is detachably connected to a video processor. In the video processor, an output signal from the CCD is A/D converted, white balance is adjusted, an NTSC video signal from a post-process circuit is subjected to processing for generating a basic video signal to be outputted to a monitor, and an option substrate having a connector detachable to a connector for outputting a signal after AGC processing is attached thereto. Thus, user's desired function such as output to the monitor by emphasis processing can easily be realized by attaching the option substrate or the like.