A nitrogen trapping agent is added by a metering hopper to raw materials, which are melted and poured into a mold cavity of a metal mold device (18). The molten metal is then cooled and solidified into a casting (B). Before the molten metal is fully cooled and solidified, free nitrogen present in the molten metal is quickly trapped by the nitrogen trapping agent, producing a nitride. Then, a chilled structure generated in the surface layer of the casting (B) is decomposed by heating the casting (B). Finally, the heated casting (B) is machined into a camshaft (A) which is a cast iron member as a final product.