In a casting process using a resin core, sometimes molten resin remains in a cast product after the steps of withdrawal of the resin core from the cast product. In order to solve the problem, as a material of the resin core, a resin which is hard and not deformed against high temperature and high pressure of molten metal until the molten metal is solidified and is softened with an increase of temperature beyond the temperature at which the metal is solidified is used. The resin core is withdrawn from the cast product after it is softened but before it is melted. The softened core is pulled out from the cast product without being broken apart.