An ultrasound treatment instrument, comprising: an ultrasound probe that transmits ultrasound vibration; a treatment portion provided at a distal end side of the ultrasound probe for treating a subject by the ultrasound vibration; a sheath through which the ultrasound probe is inserted; a clamp arm that is pivotally supported swingably at the sheath via a shaft portion; a base that is provided at the clamp arm in a position separated to a distal end side from the shaft portion, and displaces between a closed state being in close proximity to the ultrasound probe and an opened state separated from the ultrasound probe by being linked with a swing action of the clamp arm; a jaw that is supported swingably around an axis extending in a width direction of the base to extend in a longitudinal direction of the base, and has a proximal end portion located at a proximal end side from a distal end portion of the base to form a space between the proximal end portion and the base; and operation portions (28, 58) for operating the jaw to move between a closed state in which the jaw is in close proximity to the treatment portion and an opened state in which the jaw is separated from the treatment portion; and further comprising: stoppers having rigidity that are integrally formed on the sheath and extend toward side portions of the jaw, and are formed to be arranged at side portions of the jaw and the base to cover the space when the jaw is moved from the opened state to the closed state; and auxiliary stoppers that are provided protrudingly from the base to face side portions of a proximal end side of the jaw. an ultrasound probe that transmits ultrasound vibration; a treatment portion provided at a distal end side of the ultrasound probe for treating a subject by the ultrasound vibration; a sheath through which the ultrasound probe is inserted; a clamp arm that is pivotally supported swingably at the sheath via a shaft portion; a base that is provided at the clamp arm in a position separated to a distal end side from the shaft portion, and displaces between a closed state being in close proximity to the ultrasound probe and an opened state separated from the ultrasound probe by being linked with a swing action of the clamp arm; a jaw that is supported swingably around an axis extending in a width direction of the base to extend in a longitudinal direction of the base, and has a proximal end portion located at a proximal end side from a distal end portion of the base to form a space between the proximal end portion and the base; and operation portions (28, 58) for operating the jaw to move between a closed state in which the jaw is in close proximity to the treatment portion and an opened state in which the jaw is separated from the treatment portion; and further comprising: stoppers having rigidity that are integrally formed on the sheath and extend toward side portions of the jaw, and are formed to be arranged at side portions of the jaw and the base to cover the space when the jaw is moved from the opened state to the closed state; and auxiliary stoppers that are provided protrudingly from the base to face side portions of a proximal end side of the jaw. stoppers having rigidity that are integrally formed on the sheath and extend toward side portions of the jaw, and are formed to be arranged at side portions of the jaw and the base to cover the space when the jaw is moved from the opened state to the closed state; and auxiliary stoppers that are provided protrudingly from the base to face side portions of a proximal end side of the jaw.