A holding device includes a cylindrical main body portion having an outer cross-section equal to or less than a cross-section defining the inner wall of an upper portion and a pair of locking pieces protruding outward from the distal end portion of the main body portion in the direction of the central axis along a direction perpendicular to the central axis at the positions of a pair of slots while the main body portion is positioned within the inner wall of the upper portion when viewed from the direction of the central axis. The pair of locking pieces have such a shape as to be contained within the outer shape of the head portion while being positioned within the pair of slots.
