An endoscope forceps stopcock has a cylindrical body. The body has a lower engagement section detachably fitted to an insertion mouthpiece of an endoscope, and an upper engagement section to which a cap is detachably fitted. The body and cap are connected to each other by a coupling member. A valve section is formed in the body between the engagement sections to close an insertion channel and provided with a pushing section and an insertion section allowing for the insertion of medical operation instruments. The pushing section presses the cap in accordance with the extent of the deformation of the valve section when the pressure in the insertion channel is increased, thereby holding the cap in cooperation with the upper engagement section.