PURPOSE: To provide a condensate comprising a vinyl aromatic polymer, a styrene-diene block copolymer/oil blend and a blowing agent, which condensate lowers melt viscosity when added to a thermoplastic organic polymer and gives a lightweight article having a good cell structure.

CONSTITUTION: There is provided a blowing agent condensate comprising (A) a vinyl aromatic polymer, (B) a preformed blend of a styrene/diene block copolymer with a hydrocarbon rubber extending oil and (C) a blowing agent. As the component A is employed one selected from a homopolystyrene, a poly-α- methylstyrene, a styrene-maleic anhydride copolymer and a styrene-acrylonitrile copolymer. The component A can be a homopolystyrene modified with a rubber or with butadiene. As the oil is employed a paraffinic oil or a naphthenic oil. As the blowing agent is employed an azodicarbonamide, a hydrazodicarboxylate or the like. This condensate is adapted for a thermoplastic resin such as a polyimide, a polyphenylene sulfide or the like.