A treatment instrument that is inserted in a freely advancing and retracting manner into the channel of an ultrasonic endoscope, includes a treatment member that has a distal end and a proximal end and an outer surface that is a circular cylindrical shape in which a plurality of dimples are formed indenting into the outer surface. The length component that is measured in the direction of the center axis of the treatment member and which is the distance between the rear edge, which is positioned closest to the proximal end, and the deepest part of the dimple is shorter than the length component that is measured in the direction of the center axis and which is the distance between the front edge which is closest to the distal end and the deepest part.
