An ultrasonic surgical instrument includes an elongated shaft having respective proximal and distal end sides. An ultrasonic transducer is secured to the shaft and is configured to transmit ultrasonic vibrations from the proximal end side toward a distal end side along a longitudinal axis of the shaft. A cutting portion includes an outermost shape-defining portion that when pressed at a tip of the block shape against a bone cuts the bone in a direction of a pressing force to form a bone tunnel and to produce small particle of bone. A burying portion is configured to direct a flow of the fluid, which contains the small particle of bone produced upon formation of the bone tunnel, toward a wall formed in the bone tunnel by the cutting portion along the direction of the pressing force and to bury the small particle of bone in the wall of the bone tunnel.