A processing aid (B) for foam molding use which comprises a powdered copolymer composed of 50 to 80% by weight of methyl methacrylate, 20 to 50% by weight of an alkyl methacrylate having an alkyl group of 3 to 5 carbon atoms, and 0 to 20% by weight of other copolymerizable monomers, the copolymer having a reduced viscosity (&eegr;sp/c) of 4 to 8. Use of this processing aid permits vinyl chloride resins to be kneaded efficiently and imparts an increased melt strength thereto, so that fine and uniform foamed cells can be formed and, moreover, a high foaming ratio and an attractive molding appearance can be achieved.