The disclosed technology is directed to a treatment instrument that comprises a sheath main body having opposed respective distal and proximal end regions. The sheath main body includes an outer diameter such that the sheath main body capable of being fit into the channel for insertion of the treatment instrument of the endoscope. The sheath main body includes a lumen extending along a longitudinal axis in the distal end region. A wire-shaped incising portion is configured to engage with the sheath main body so as to protrude from an outer circumferential surface of the sheath main body in the distal end region and extending along the longitudinal axis of the sheath main body. The wire-shaped incising portion is used to incise a tissue. A wire is attached to at least a portion of the sheath main body and having a region in which the wire has different bending resistances.