A cylindrical rotation unit provided outside an elongated insertion body which defines a central axis being substantially parallel with a longitudinal axis of a channel having an opening end portion in which a rotation gear having external teeth of a rotation force transmission unit is detachably provided and which is open to the outside, includes an internal gear which includes internal teeth configured to mesh with the external teeth of the rotation gear and which is rotatable around the central axis while the rotation gear is disposed in the opening end portion of the channel, and a tube which is configured to support the internal gear on an inner circumferential surface of the tube.
