In an electronic endoscope system, an RF signal is produced through quadrature modulation of a picture signal that is representative of an image taken through an electronic endoscope. When control signals are entered by an operator through a control section of the electronic endoscope, a data superimposing section superimposes the entered control signals on the RF signal in horizontal scanning intervals within a vertical blanking interval. The RF signal having the control signals superimposed thereon is sent as an electric wave of a single frequency band to a processor. In the processor, a data analyzer carries out sampling to extract the entered control signals if they are superimposed on the picture signal, and analyzes the contents of the entered control signals. Based on the results of analysis, a CPU controls respective components of the signal processor.