An electronic endoscope system comprises an electronic endoscope and a processor. The electronic endoscope includes a parallel/serial converter which encodes a vertical synchronizing signal and a horizontal synchronizing signal to synchronization codes representing ON/OFF states thereof expressed by several maximum and minimum signal levels for representing the digital image signals, and uses a maximum and a minimum signal levels except the signal levels representing the synchronization codes to express the digital image signals which would normally be represented by the signal levels representing the synchronization codes, while the image signals whose levels are not in the signal levels representing the synchronization codes are not subject to change. The processor includes a synchronizing signal decoder for decoding the vertical synchronizing signal and the horizontal synchronizing signal based on the synchronization codes.