There is provided a treatment instrument, which is provided with a flexible sheath configured such that at least a part thereof is formed of a flexible tube, an operation wire inserted into the flexible sheath to be movable in an axial direction of the flexible sheath, a front-end treatment member connected to a leading end of the operation wire so that the front-end treatment member is allowed to protrude from a front end of the flexible sheath and retracts into the front end of the flexible sheath in accordance with movement of the operation wire, and an at least one elastic member that has elasticity and is located in a front end portion of the flexible sheath in a condition where the at least one elastic member is elastically deformed between an inner surface of the flexible sheath and an outer circumferential surface of the operation wire.