An apparatus for transmitting liquid and gas in an endoscope comprising conduits for transmitting liquid and gas to an observation optical system to clean the observation optical system; and an automatic change-over device for sequentially operating both modes of the liquid and gas transmissions by turning on and off only a liquid transmitting switch disposed in an operating portion. The automatic change-over device comprises a first electromagnetic valve disposed in the liquid transmitting conduit, a second electromagnetic valve disposed in the gas transmitting conduit, a circuit for connecting the first and second electromagnetic valves to the liquid transmitting switch of the operating portion, and a control section for controlling the operations of the electromagnetic valves. When the liquid transmitting switch is turned on, the first electromagnetic valve is opened to transmit the liquid to the liquid transmitting conduit, and when the liquid transmitting switch is turned off, the first electromagnetic valve is automatically closed and the second electromagnetic valve is automatically opened to transmit the gas to the gas transmitting conduit for a constant or predetermined time.