PURPOSE: To provide the titled composition giving a foamed article having fine cells, by adding a specific aliphatic dicarboxylic acid diamide to a polymer of a vinyl monomer, and impregnating the polymer with a foaming agent during the latter half stage of polymerization or after the polymerization.

CONSTITUTION: The objective resin composition is prepared by (1) adding preferably ≥0.005wt% of an aliphatic dicarboxylic acid diamide of formula (R and R' are 6W24C bivalent aliphatic hydrocarbon group; n is 1W8) (e.g. N,N'-dioleyladipic acid amide) to the suspension polymerization system of a vinyl monomer (preferably styrene or its derivative) in an aqueous medium before the polymerization conversion of said monomer reaches 50%, and (2) impregnating the polymer with preferably 1W20% of a foaming agent (e.g. propane) during the latter half stage of the polymerization or after the completion of polymerization.

EFFECT: Since the composition does not cause cell rupture even if exposed to high temperature, it can be stored without necessitating a cooling apparatus.