A resin composition is described that is excellent in strength, impact resistance, heat resistance and moldability, while further allowing for a reduction in the amount of CO2 needed for its production. The resin composition includes a styrene-based resin (A), an aliphatic polyester (B) and at least one species selected from a compatibilizer (C) and a dicarboxylic anhydride (D), wherein the compatibilizer is preferably at least one species selected from:(C-1) a methyl methacrylate polymer; (C-2) a vinyl-based polymer to which an epoxy unit or acid anhydride unit is copolymerized; (C-3) a graft polymer in which a methyl methacrylate unit is grafted to a rubbery polymer; and (C-4) a block copolymer having a polylactide segment and a vinyl-based polymer segment.