An attachment unit includes a tube main body extended along a longitudinal axis in a state that a gap is provided between itself and an outer peripheral portion of an insertion section or a member on the outer peripheral portion of the insertion section, and a tube proximal end portion, provided to a proximal direction side of the tube main body, to which a proximal end of the attachment unit is placed. The tube proximal end portion includes a proximal side gap reducing portion eliminating or reducing the gap as compared with a part to an inner peripheral direction side of the tube main body between itself and the outer peripheral portion of the insertion section or the member, and is placed to a distal direction side of an insertion opening in a use situation.
