An attachment unit rotates in directions around a longitudinal axis with respect to an insertion main body when rotary drive force is transmitted from a drive force transmission unit provided to an insertion section, and allows propulsive force in one of directions parallel to the longitudinal axis to act on the insertion section by rotating in contact with a paries. The attachment unit includes a fin portion which is spirally extended along the longitudinal axis and whose dimension from the longitudinal axis to an outer peripheral end vary in accordance with how external force different from the propulsive force acts in the directions parallel to the longitudinal axis in a state that the attachment unit does not rotate with respect to the insertion main body.