To improve the collapsibility of a mold after pouring molten metal while holding the mold characteristics without practically developing any trouble by using a phenolic resin and incorporating a sodium phosphate compound as a collapsibility improver.

The collapsibility improver is contained in the ratio of 0.5-10 pts.wt. as effective component (Na3PO4 or Na2HPO4) per 100 pts.wt. of the phenolic resin. This containing ratio is desirable to be 1-5 pts.wt. For blending the collapsibility improver, any method which beforehand blends it with the phenolic resin, charges it in a kneading process for applying the phenolic resin on molding sand or dispersely mixes it into the molding sand particles coated with the phenolic resin, can be adopted. As the sodium phosphate compound, a hydrate and an anhydrous compound, of disodium phosphate and trisodium phosphate or the like are given. This improver is effective in the case of casting a low m.p. metal, such as aluminum or magnesium alloy.