A plastic forming die (20) for formation of outer surface is set on the inner surface of a cavity of a die. The forming die has a forming die body in a square barrel shape, and there are a lot of through holes (22) provided between mount surfaces (25) and inner side surfaces (26) of the forming die body. Grooves (23) communicating with the through holes (22) are provided in the mount surfaces (25). When a molten metal is injected into the cavity, the through holes of the forming die body form outwardly projecting portions of a cast product. The forming die is then melted to be removed from the cast product.