In a treatment instrument, one of a pair of jaws is supported by a supporter while being pivot able around the supporter with respect to the supporter. A drive member moves along a longitudinal axis so that the pair of the jaws are closed with respect to each other, thereby applying an axial force onto an elongated member from the drive member by way of the one of the jaws and the supporter in a direction along the longitudinal axis. A contact surface of the elongated member abuts on a housing when the elongated member moves with respect to the housing under the axial force, thereby suppressing rotation of the elongated member around the longitudinal axis with respect to the housing.