An endoscope system (2) has an electronic endoscope (10) with a solid-state image sensor (40), a processor device (11, 91), and an option circuit board (56, 92) detachably connected to the processor device. The electronic endoscope drives the solid-state image sensor and captures images. The processor device generates a first video signal from an image signal. The option circuit board has a measurement pulse generator (81) for generating measurement pulses, a gate pulse generator (82) for generating a gate pulse, a pulse counter (83) for counting the number of the measurement pulses during outputting the gate pulse, and a judgment circuit (84) for obtaining the number of pixels from a count value of the pulse counter. The option circuit board converts the first video signal into a second video signal in consideration of the number of pixels to display the images on a PC monitor (87).
