A joint ring includes a main body part 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 main body part includes a pair of engagement holes and a pair of receiving holes. The protrusion parts include engagement parts which are configured to be capable of being inserted into the engagement holes. When the joint rings are arranged in the direction of the central axis and the protrusion parts of a first joint ring enter the receiving holes of a second 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 as a rotational center.
