PURPOSE: To greatly improve the collapsing property of a casting mold without impairing not only the free flow property of molding sand but the characteristics such as strength, of the casting mold such as core, as well by adding a peroxide to an phthalyl type polymerizable material and dispersing the mixture composed thereof into a specific amt. of an org. solvent.

CONSTITUTION: The phthalyl type polymerizable material such as diallyl orthophthalate, is adequately low in thermal decomposition rate and, therefore, such a problem that blowholes are formed in a casting by the gases generated before solidification of molten metal is suppressed as far as possible. A dicumyl peroxide, etc., are used as the peroxide to be used as a hardener and is usually added at 1 to 7% to the phthalyl type polymerizable material as the compounding ratio thereof. The org. solvent which is a medium of the binder for the molding sand is a relatively low b.p. solvent, such as methyl ethyl ketone, which can solve the phthalyl type polymerizable material and the peroxide. The compounding ratio of a methyl ethyl ketone, etc., is 1/3 to 1 of 1 pt.wt. mixture composed of the phthalyl type polymerizable material and the peroxide.