To surely wash and disinfect all parts without forming parts not to be in contact with a liquid chemical at all inside a conduit inside an endoscope and inside a mouth ring fixed to an entrance of the conduit inside the endoscope.

A washing adapter 2 for an endoscope is used by being mounted on a mouth ring 24A fixed to the entrance of the conduit inside the endoscope, and a guide pipe 58 inserted to the inside of the mouth ring 24A is vertically moved according to the pressure fluctuation of a washing liquid. A thick diameter part 58BG in a truncated conical shape whose outer diameter is reduced on a lower end side is provided on a lower end part of the guide pipe 58, and the outer peripheral surface of the thick diameter part 58BG is close to the inner wall of the mouth ring 24A. The thick diameter part 58BG is provided with a main stream exit 58BH for discharging the washing liquid that flows in toward the center of the mouth ring 24A, and a shunt exit 58BI for discharging the washing liquid toward an upper part.