An ultrasonic treatment instrument is provided with an ultrasonic transducer, a probe portion to which ultrasonic wave outputted from the ultrasonic transducer is transmitted, a sheath portion into which the probe portion is inserted detachably, the sheath portion including at a distal end portion thereof, a grasping member rotatably supported so as to face the probe portion, and a handle portion coupled to a proximal end portion of the sheath portion detachably and operating the grasping member such that the grasping member is opened and closed to the probe portion, where the distal end portion of the probe portion includes a cavitation production portion where the distal end portion of the probe portion has, on a face thereof facing the grasping member, a flat face extending so as to intersect a vibration direction of the ultrasonic vibration, and cavitation is produced by the flat face in a state that a body tissue has been grasped between the grasping member and the distal end portion of the probe portion.
