To solve such problems that in the case foreign matter exists on the surface of a chaplet held between a core and a mold, melting point is lowered and fusion with molten metal is increased and also, leakage can be restrained but the foreign matter is vaporized, burned and washed away with high temperature molten metal at pouring time only by applying, plating or vapor-depositing on the chaplet surface, and in the case of attaching a flange for preventing leakage, large labor is needed in this work and in the case adhesion with the molten metal is bad, the molten metal is leaked.

A pressure-proof chaplet for casting is constituted by diffusion-penetrating a material having the foreign matter onto the chaplet while applying the diffusion-penetration of carburizing treatment, electrolytic carburizing, ion-carburizing, carbonitriding, aluminizing, sulfurizing, chromizing, siliconizing, sheradizing, boronizing, ion-plating, sputtering, etc., on the chaplet surface. In this way, since the foreign matter is diffusion-penetrated on the chaplet surface, this matter is not vaporized, burned and washed away, the melting point is lowered with lower of solidifying point, and the fusion with the molten metal can be increased and the leakage can be prevented.