An ultrasonic treatment instrument for use in arthroscopic surgery, comprising: a probe having: a main body that is configured to transmit ultrasonic vibration, and a treatment portion that is provided on a distal side of the main body and is configured to cut a hard tissue and/or a soft tissue by transmission of the ultrasonic vibration, the main body prescribing a central axis, the treatment portion including: a bent portion bent from a distal end of the main body that is continuous with the main body and is bent from a distal end of the main body, and a cutting region provided on a distal side of the bent portion and formed at a position shifted from the central axis due to the bent portion; and a sheath unit that includes: an inner sheath including a distal portion which covers the main body present in a region on a proximal side of the bent portion in the probe and covers a region bent from the distal end of the main body in a proximal portion of the bent portion, and an outer sheath covering an outer peripheral surface of the inner sheath and forming a suction path between the outer peripheral surface of the inner sheath and the outer sheath, wherein a distal end of the inner sheath extends to a position distally closer to the cutting region of the treatment portion than a distal end of the outer sheath, and through-holes communicating between an outer peripheral surface of the outer sheath and an inner peripheral surface of the outer sheath are formed in a distal portion of the outer sheath. a probe having: a main body that is configured to transmit ultrasonic vibration, and a treatment portion that is provided on a distal side of the main body and is configured to cut a hard tissue and/or a soft tissue by transmission of the ultrasonic vibration, a main body that is configured to transmit ultrasonic vibration, and a treatment portion that is provided on a distal side of the main body and is configured to cut a hard tissue and/or a soft tissue by transmission of the ultrasonic vibration, the main body prescribing a central axis, the treatment portion including: a bent portion bent from a distal end of the main body that is continuous with the main body and is bent from a distal end of the main body, and a cutting region provided on a distal side of the bent portion and formed at a position shifted from the central axis due to the bent portion; and a sheath unit that includes: an inner sheath including a distal portion which covers the main body present in a region on a proximal side of the bent portion in the probe and covers a region bent from the distal end of the main body in a proximal portion of the bent portion, and an outer sheath covering an outer peripheral surface of the inner sheath and forming a suction path between the outer peripheral surface of the inner sheath and the outer sheath, a bent portion bent from a distal end of the main body that is continuous with the main body and is bent from a distal end of the main body, and a cutting region provided on a distal side of the bent portion and formed at a position shifted from the central axis due to the bent portion; and a sheath unit that includes: an inner sheath including a distal portion which covers the main body present in a region on a proximal side of the bent portion in the probe and covers a region bent from the distal end of the main body in a proximal portion of the bent portion, and an outer sheath covering an outer peripheral surface of the inner sheath and forming a suction path between the outer peripheral surface of the inner sheath and the outer sheath, an inner sheath including a distal portion which covers the main body present in a region on a proximal side of the bent portion in the probe and covers a region bent from the distal end of the main body in a proximal portion of the bent portion, and an outer sheath covering an outer peripheral surface of the inner sheath and forming a suction path between the outer peripheral surface of the inner sheath and the outer sheath, wherein a distal end of the inner sheath extends to a position distally closer to the cutting region of the treatment portion than a distal end of the outer sheath, and through-holes communicating between an outer peripheral surface of the outer sheath and an inner peripheral surface of the outer sheath are formed in a distal portion of the outer sheath.