In a case of a “mark left” type, a slit is formed on a line side (slit side) of a joint portion. The slit is formed, for example, with a scroll saw. By the formation of the slit, a length of the joint portion in a direction orthogonal to an axial direction of a single ring is set to approximately 0.7 mm. In a case of a “mark removed” type, a slit and a slit are formed respectively on the line side (slit side) and a side opposite thereto of the joint portion. Those slits are formed, for example, with the scroll saw. By the formation of those slits, the length of the joint portion in the direction orthogonal to the axial direction of the single ring is set to approximately 0.8 mm.