A joint ring includes a main body part formed in an annular shape and a pair of protrusion parts protruding from a first surface of the main body part in a direction of a central axis thereof and having spherical surfaces on protruding portions. The pair of protrusion parts are formed to oppose each other across the central axis. The main body part includes a pair of receiving holes which are open in a second surface in the direction of the central axis, have sizes such that the spherical surfaces are configured to be capable of entering the receiving holes, and are formed to oppose each other across the central axis. When a plurality of the joint rings are arranged in the direction of the central axis and the pair of protrusion parts of a first joint ring enter the pair of receiving holes of a second joint ring adjacent to the first joint ring, the spherical surfaces come into contact with receiving surfaces inside the receiving holes, and the first joint ring and the second joint ring become rotatable relative to each other with a line segment connecting centers of the respective spherical surfaces of the pair of protrusion parts as a rotational center.
