PURPOSE: To reduce the diameter of a sheath by preventing a liquid leak at the time of liquid feed or an air leak at the time of suction by hermetically covering the sheath part coming out of the channel port part of an endoscope.

CONSTITUTION: Bellows 10 constitutes a freely expansible cover means hermetically covering a sheath 2 and a rubber plug 13 hermetically connected to the cap part of the insertion port part 12 of the treatment device inserting channel of an endoscope in a freely detachable manner is provided to the other end of the bellows 10. When a contrast medium is fed, the bellows 10 is extended to fit the leading end rubber plug 13 in the cap part of the insertion port part of the endoscope 11 to connect the same to the cap part in a liquidtight manner and, therefore, the exposed part of the sheath 2 coming out of the insertion port part 12 of the endoscope 11 is perfectly covered with the bellows 10. As a result, a liquid leak is eliminated at the time of liquid feed and a liquid can be certainly fed out. Further, even when the liquid in the gall bladder 15 is absorbed, the open air is not sucked from the part of the sheath 2 and the lowering of suction force can be prevented.