A resin composition having excellent impact resistance and heat resistance, comprising a poly-3-hydroxybutyrate, and a core-shell latex rubber comprising an acrylic rubber and/or silicone-acrylic rubber copolymer as a core component and polymethyl methacrylate as a shell component, or a specific thermoplastic polyurethane, is disclosed. The resin composition satisfies the following requirements (c) and (d): (c) a crystallization temperature when heated from room temperature to 180.degree. C. at a temperature rising rate of 80.degree. C./min by a differential scanning calorimeter, maintained at 180.degree. C. for 1 minute, and then cooled at a temperature lowering rate of 10.degree. C./min is 110-170.degree. C.; and (d) a weight average molecular weight (Mw) in terms of polystyrene conversion when a chloroform soluble component is measured with a gel permeation chromatography is 100,000-3,000,000.