A treatment instrument for medical use includes a tubular insertion portion that includes a curvable flexible part, the tubular insertion portion being insertable into a body of a patient； and a dilating body that is provided at an outer circumference of the flexible part, the dilating body being configured to dilate in a radial direction relative to the flexible part. The tubular insertion portion includes a reduced-diameter part that is provided at a distal end of the tubular insertion portion in an axial direction, the reduced-diameter part having an outer diameter that is smaller than an outer diameter of the tubular insertion portion at a location proximal of the reduced-diameter part. A distal part of the dilating body is fixed to at least the reduced-diameter part.
