An electronic endoscope system for observing living tissues inside a body cavity includes an illuminating apparatus having a white light source emitting white light and an excitation light source emitting excitation light, an electronic endoscope including an insertion part to be inserted into the body cavity, an imaging device that receives an optical image and outputs image signals corresponding to the optical image, an image forming system that forms the optical image, an operating pant provided at a rear anchor of the insertion part, and a plurality of operable members arranged at the operating part, a display device, an image processing system that receives the image signals outputted from the imaging device, the image processing system obtaining a normal image when the living tissues are illuminated with the white light and a fluorescence image when the living tissues are irradiated with the excitation light, and a control system that controls the whole of the electronic endoscope system. The control system assigns a function to one of the plurality of operable members for switching between a normal mode for observing the normal image and a fluorescence mode for observing the fluorescence image. The control system assigns different functions between the normal mode and fluorescence mode to at least one of the others of the plurality of operable members.