An electronic endoscope system with high operability and high observing performance capable of driving properly and automatically the light-emitting elements without the size increase in system is provided, even in the case of using a plurality of interchangeable optical adaptors having varied numbers of light-emitting elements. A plurality of optical adaptors having LEDs are detachable to the distal end of an insertion unit of an endoscope. The insertion unit has an inserted electric cable connected to the LEDs. A constant-current circuit of an LED driving circuit flows constant current to the LEDs via the electric cable.