The present invention relates to a copolymer comprising 4-hydroxybutyrate monomer unit and lactate monomer unit, a copolymer 4-hydroxybutyrate monomer unit, lactate monomer unit and 3-hydroxyalkanoate, or their preparing method. More specifically, the present invention relates to a method for preparing a copolymer comprising lactate monomer; 4-hydroxybutyrate monomer; and optionally 3-hydroxyalkanoate, wherein the method comprises culturing a cell or plant comprising the gene of enzyme converting lactate and 3-hydroxyalkanoate into lactyl-CoA and 3-hydroxyalkanoyl-CoA, respectively, phosphotransbutylase gene, butyrate kinase gene and polyhydroxyalkanoate synthase gene together, and the copolymer made by the method. The copolymer of the present invention is a biodegradable polymer being able to be usefully used instead of conventional synthetic plastic, and the copolymer can be used for medical use.