An ultrasonic treatment instrument for use in arthroscopic surgery, includes 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 tis e by transmission of the ultrasonic vibration； and a sheath unit that includes an inner sheath covering an outer peripheral surface of the main body in the probe, 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 closer to the treatment portion of the probe than a distal end of the outer sheath.
