PROBLEM TO BE SOLVED: To provide a medical treatment instrument which can prevent an expansion body from coming off an insertion part when inserting the expansion body in a body.SOLUTION: A medical treatment instrument 1 includes: a pipe-shaped insertion part 2 at least a part of which is composed of a bendable flexible part 21 and which is inserted in a patient's body； and an expansion body 5 which is provided for an outer periphery of the flexible part 21 and expands in a radial direction of the flexible part 21. The insertion part 2 is provided with a diameter reduction part 22 which is provided at an end part in an axial direction of the insertion part 2 and whose outer diameter is reduced toward the end part. The end part 51 in the axial direction of the expansion body 5 is fixed to at least the diameter reduction part 22.【課題】体内に拡張体を挿入する際に拡張体が挿入部から外れてしまうことを防止できる医療用処置具を提供すること。【解決手段】医療用処置具１は、少なくとも一部が湾曲可能な可撓部２１で構成され、患者の体内に挿入される管状の挿入部２と、可撓部２１の外周に設けられ、当該可撓部２１の径方向に拡張する拡張体５とを備え、挿入部２は、当該挿入部２の軸方向の先端部に設けられ、当該先端部に向かって外径が縮径した縮径部２２を備え、拡張体５の軸方向の先端部５１は、少なくとも縮径部２２に固定されていることを特徴とする。【選択図】図１
