An ultrasonic surgical instrument (1) has an ultrasonic transducer unit (12) having an ultrasonic transducer (12b) for producing ultrasonic vibration, a transmission member (30) which connects the ultrasonic transducer (12b) in the proximal end side, and transmits the ultrasonic vibration produced by the ultrasonic transducer (12b) from the proximal end side to the distal end side, a sheath (20), in which the transmission member (30) is inserted, having an open part (25) on one side of the distal end side, and a treatment portion (31) which is provided at the distal end of the transmission member (30), and exposed to the open part (25) for carrying out a surgical treatment on an object part by the ultrasonic vibration transmitted from the transmission member (30). The ultrasonic surgical instrument (1) cuts or shaves hard tissues.
