PURPOSE: To compensate the decrease in the casting strength owing to defective packing and to prevent the generation of casting defects in casting using an expanded resin pattern by forming a backup layer of a prescribed thickness in the places of the pattern where molding sand may be unsatisfactorily packed.

CONSTITUTION: A known mold coating material layer 10 is first formed on the surface of an expanded resin pattern 12 in the stage of investing the pattern 12 with molding sand 4, charging molten metal in the pattern and expending the pattern 12. A backup layer 14 of a prescribed thickness is formed on the layer 10 in the part 8 behind the packing direction A of the molding sand. A slurry contg. pulverized powder of refractories (for example, silica sand, etc.) and a binder (for example, tetraethyl silicate, etc.) is coated plural times on the layer 14, preferably to about several mmW10mm thickness. The substantial strength of the casting mold is thus assured and the problems such as swell in the stage of charging are eliminated.