A treatment instrument includes: a proximal side probe transmits ultrasonic vibration from an ultrasonic transducer from its proximal end to its distal end； a distal side probe configured to transmit the ultrasonic vibration from its proximal end to a treatment portion at its distal end； a holding portion which holds a state where the proximal end of the distal side probe is into collision with the distal end of the proximal side probe, and a state where vibration from the ultrasonic transducer is transmittable from the proximal end of the proximal side probe to the distal end of the distal side probe； and an operation portion provided in the holding portion and rotating the distal side probe relative to the proximal side probe around a central axis by an operation to rotate the distal side probe around the central axis, to direct the treatment portion at the distal end of the distal side probe in a desired direction.
