A treatment instrument that is inserted in a freely advancing and retracting manner into the a channel of an ultrasonic endoscope that includes an ultrasonic scanning mechanism emitting and receiving ultrasound waves, the first dimple includes a first area which has a reflecting surface that reflects the ultrasound waves emitted from the ultrasonic scanning mechanism to the ultrasonic scanning mechanism and a second area which is arranged contiguously to the first area at a proximal end side of the first area and which includes a wall surface on which the ultrasound waves that is emitted toward the first area in a direction to travel directly from the ultrasonic scanning mechanism cannot be incident.
