A treatment device includes a narrow main unit, an end effector, a rolling joint provided between the end effector and the main unit, a driven joint that is provided at the end effector or between the end effector and the rolling joint and that rotates together with the rolling joint about a longitudinal axis, power transmission members that extend parallel to each other from the driven joint to the main unit, position maintaining portions that maintains the power transmission members at two positions located between the driven joint and the main unit and separated from each other by a certain distance in a direction of the longitudinal axis, and a shaft member disposed along the longitudinal axis between the two positions. The position maintaining portion maintains the power transmission members at predetermined relative positions separated from each other by the certain distance in a circumferential direction of the shaft member.
