A treatment device (1) according to the present invention includes a narrow main unit, an end effector (3), a rolling joint (8) provided between the end effector (3) and the main unit, a driven joint (7) that is provided at the end effector (3) or between the end effector (3) and the rolling joint (8) and that rotates together with the rolling joint (8) about a longitudinal axis (A), a plurality of power transmission members (10A, 10B) that extend parallel to each other from the driven joint (7) to the main unit, a position maintaining means (13, 14) that that maintains the power transmission members (10A, 10B) at two positions located between the driven joint (7) and the main unit and separated from each other by a certain distance in a direction of the longitudinal axis (A), and a shaft member (12) disposed along the longitudinal axis (A) between the two positions. The position maintaining means (13, 14) maintains the power transmission members (10A, 10B) at predetermined relative positions separated from each other by the certain distance in a circumferential direction of the shaft member (12).
