The present invention provides a resin composition excellent in strength, impact resistance, heat resistance and moldability and further, of low environmental load capable of decreasing CO.sub.2 exhaustion in its production. The present invention is a resin composition comprising a styrene-based resin (A), an aliphatic polyester (B) and at least one species selected from a compatibilizer (C) and a dicarboxylic anhydride (D), and the compatibilizer is preferably at least one species described below. (C-1) methyl methacrylate polymer (C-2) a vinyl-based polymer to which epoxy unit or acid anhydride unit is copolymerized (C-3) a graft polymer in which methyl methacrylate unit is grafted to a rubbery polymer (C-4) a block copolymer in which polylactide segment and a vinyl-based polymer segment