A medical treatment instrument includes a sheath, a needle advanceably and retractably inserted into the sheath, and a stylet having a first outer diameter and advanceably and retractably inserted into the needle. A distal end portion of the needle includes a housing portion, a small-diameter portion provided at a proximal end of the housing portion and having an inner diameter smaller than that of the housing portion, and a slit extending at least from a distal end of the housing portion to a proximal end of the small-diameter portion. A distal end portion of the stylet includes a large-diameter portion having a second outer diameter larger than the first outer diameter. The small-diameter portion and the housing portion are elastically deformed to expand in a radial direction of the needle while a width of the slit expands by the large-diameter portion being engaged with the small-diameter portion.
