To eliminate any possibility of a reuse of a plug body without forming a low strength part or the like at a part of the plug body.

This forcepts cap 21 is formed with an insertion hole 33 in which a ferrule 20 is inserted, inside the cylindrical plug body 29 thereof. The plug body 29 includes, around the inner peripheral surface thereof, four engagement parts 30 engaged with a flange 25 formed around the outer peripheral surface of the ferrule 20. The engagement part 30 is provided with an engagement claw 37 engaged with the flange 25, and a projection part 38 projecting towards the axis of the insertion hole 33. A valve element 31 with a slit 43 is provided on the inner peripheral surface of the plug body 29 at an inlet side of a treatment device 14 with respect to each engagement part 30. When the forcepts cap 21 is removed, an engagement release device 47 is inserted into the insertion hole 33. The engagement release device 47 breaks the slit 43 and then, depresses the projection parts 38 of the respective engagement parts 30 to release the engagement between the respective engagement claws 37 and the flange 25. The slit 43 is broken so as to eliminate any possibility of the reuse of the forcepts cap 21.