Attachment unit and endoscope

An attachment unit being configured to be attached to an insertion section which includes an insertion main body extended along a longitudinal axis and which is configured to be inserted into a lumen, and being provided to an outer peripheral direction side of the insertion section while being rotatable in directions around the longitudinal axis with respect to the insertion main body,

• wherein the attachment unit is configured to rotate in the directions around the longitudinal axis with respect to the insertion main body when rotary drive force is transmitted from a drive force transmission unit provided to the insertion section, and is configured to allow propulsive force in one of directions parallel to the longitudinal axis to act on the insertion section by rotating in contact with a paries, and
• 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 is configured to 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.