An electronic endoscope has an image sensor for converting an optical image into analog electric image-pixel signals, and an analog-to-digital converter for converting the analog electric image-pixel signals into digital electric image-pixel signals on the basis of a first series of clock pulses. A number of the converted digital electric image-pixel signals included in a one-horizontal-scanning-line being is based upon a frequency of the first series of clock pulses. The endoscope has a pixel-number converter for converting the number of the converted digital electric image-pixel signals into another number of digital image-pixel signals so as to enable the digital electric image-pixel signals to be externally outputted on the basis of a second series of clock pulses having a frequency different from that of the first clock pulses.