PURPOSE: To easily and surely remove the filmy resin core remained inside a casting by projecting fine resin beans on the surface in contact with resin core by compressed air so as to peel/remove a thin film resin core remained after buiit-in resin core is removed from casting.

CONSTITUTION: Concerning a resin core, a core main body is formed to a prescribed shape with use of polycarbonate, etc. Also, a casting product cast with use of the resin core is used for a die casting product, etc. The resin core bult-in the casting product is removed in two processes. In a first process, the core is pulled out/removed while a temp. after casting is still high or is melted/ removed with reheating by electric furnace, etc. Successively, the thin filmy resin core remaineed inside a casting, after a temp. of casting is lowered, is peeled/removed by projecting fine resin beads on the surface in contact with resin core, specifically remained on the surface of thin film resin core by compressed air.