Abstract: In preparation of a mold for metal casting, a thermosetting resin layer, preferably of a thickness in a range from 2 to 100 micrometers, is formed between the refractory filler in the molding box and a covering laminate such as a thermoplastic synthetic resin film for tightly covering the filler surface so that the thermosetting resin layer develops, during metal casting, a shell layer for reinforcing the cavity wall of the mold due to heat given by molten metal. Development of casting defects such as sand marks and blow holes and occurrence of mold crumbling can effectively be prevented by the presence of the shell layer.