The node ring includes a main body portion formed in an annular shape, and a pair of protrusions protruding from a first surface in the central axis direction of the main body portion and having a spherical surface at the protruding portion. The pair of protrusions are formed so as to face each other with the central axis therebetween. The main body has a pair of receiving holes that are open to the second surface in the central axis direction, are sized to allow the spherical surface to enter, and are formed to face each other with the central axis therebetween. When a plurality of node rings are arranged in the direction of the central axis, and a pair of protrusions of the first node ring are inserted into a pair of receiving holes of the second node ring adjacent to the first node ring, the spherical surface becomes the receiving hole. The first and second node rings come into contact with the inner receiving surface, and the first node ring and the second node ring can be rotated relative to each other with a line segment connecting the centers of the spherical surfaces of the pair of protrusions as rotation centers.節輪は、環状に形成された本体部と、本体部の中心軸線方向における第一の面から突出し、突出部分に球面を有する一対の突起部とを備える。一対の突起部は、中心軸線を挟んで対向するように形成されている。本体部は、中心軸線方向における第二の面に開口し、球面が進入可能な大きさでありかつ中心軸線を挟んで対向するように形成された一対の受け孔を有する。節輪を中心軸線方向に複数並べ、第一の節輪の一対の突起部を、第一の節輪と隣接する第二の節輪の一対の受け孔に進入させると、球面が受け孔の内部の受け面と接触し、第一の節輪と第二の節輪とが、一対の突起部それぞれの球面の中心を結ぶ線分を回動中心として相対回動可能となる。
