A method for isolating a polyhydroxyalkanoate by extraction with an aprotic organic solvent from a biomass containing a polyhydroxyalkanoate having a weight-average molecular weight exceeding 2,000,000, wherein the weight-average molecular weight of the polyhydroxyalkanoate contained in the biomass is reduced via any of the following treatments: (a) the biomass is heated at 40-500°C before addition of the aprotic organic solvent thereto； (b) the biomass is heated at 40-500°C before addition of the aprotic organic solvent thereto, and is further heated at 40-200°C in the aprotic organic solvent； (c) the biomass is heated at 40-500°C before addition of the aprotic organic solvent thereto, and is further heated at 40-200°C in the aprotic organic solvent in the presence of water and/or an alcohol； (d) the biomass is not heated before addition of the aprotic organic solvent thereto, but heated at 40-200°C in the aprotic organic solvent； and (e) the biomass is not heated before addition of the aprotic organic solvent thereto, but heated at 40-200°C in the aprotic organic solvent in the presence of water and/or an alcohol.
