The medical treatment instrument includes a sheath, a needle inserted through the sheath so as to be able to advance and retract, and a stylet inserted through the needle so as to be advanced and retracted and having a first outer diameter. The distal end portion of the needle is provided at an accommodation portion, a proximal end of the accommodation portion, and has a small diameter portion having an inner diameter smaller than an inner diameter of the accommodation portion, and at least a proximal end of the narrow diameter portion from the distal end of the accommodation portion And a slit extending to the bottom. The tip of the stylet has a large diameter portion having a second outer diameter that is larger than the first outer diameter. When the large-diameter portion is fitted into the small-diameter portion, the width of the slit is expanded and the small-diameter portion and the accommodating portion are elastically deformed so as to expand in the radial direction of the needle.この医療用処置具は、シースと、前記シースに進退可能に挿通された針と、前記針に進退可能に挿通され、第１の外径を有するスタイレットと、を備える。前記針の先端部は、収容部と、前記収容部の基端に設けられ、前記収容部の内径より小さい内径を有する細径部と、少なくとも前記収容部の先端から前記細径部の基端まで延びるスリットと、を有する。前記スタイレットの先端部は、前記第１の外径より大きい第２の外径を有する大径部を有する。前記大径部が前記細径部に嵌合することにより、前記スリットの幅が拡大するとともに前記細径部及び前記収容部が前記針の径方向に拡張するように弾性変形する。
