Although a radial outside portion of a metal ring contact surface of a neck section of a metal element which is contactably opposed to a side edge of a metal ring of a metal ring assembly has high frequencies of contacting with a side edge of the metal ring, it is possible to suppress abrasion of the side edge of the metal ring and to improve durability of the metal ring by setting a surface hardness of the radial outside portion lower than a surface hardness of the metal ring. In addition, by setting a surface hardness of the metal ring contact surface at its portion of the radial outside lower than the surface hardness of the metal ring contact surface at other portions thereof where the frequencies of contacting with the side edge of the metal ring becomes low, in comparison with a case of setting the surface hardness of the whole metal ring contact surface low, it is possible to increase strength of the metal element and to improve the durability of the metal element.