To prevent filth, such as a highly pressurized humor injected from a throttling passage having a smaller pore size than a treatment tool inserting passage provided in an endoscope from flowing out from a clamp plug.

Although the filth, such as the humor etc., flowing from the treatment tool inserting passage 11 to the throttling passage 24, is injected with high pressure from the throttling passage 24 when a suction valve is shut off, the entrance of a slit 33 provided in a treatment tool guiding section 32 on the treatment tool inserting passage 11 side is constituted by deviating the position of the entrance from a position on the extended surface of the throttling passage 24 so that the slit 33 may not receive directly the filth, such as the humor etc., injected from the throttling passage 24.