A treatment assembly which is configured to treat a biological tissue includes a probe and a sheath. The probe defines a longitudinal axis and the sheath defines a central axis. The sheath inserts the probe in a state that a distal end portion of the probe protrudes toward a distal end side with respect to a distal end and a proximal end portion of the probe protrudes toward a proximal end side with respect to a proximal end. The sheath includes a cylindrical inner tube having an opening, which allows an inner side and an outer side thereof to communicate with each other in a side surface thereof, a support section which is arranged on the inner side of the sheath from the outer side of the sheath through the opening, and supports the probe in a state that the longitudinal axis of the probe is matched with the central axis of the sheath, and an outer tube which covers the outer side of the inner tube, and fixes the support section in a state that the longitudinal axis of the probe is matched with the central axis of the sheath.
