The re-polymerizable oligomer mixture obtained by the depolymerization of the invention can easily be re-polymerized by use of a hydrolase or a chemical catalyst. At this time, a copolymer having a varied polymer physical property can be produced by adding, to the re-polymerizable oligomer mixture, one or more monomers or oligomers selected from cyclic lactone monomers or oligomers, cyclic or linear carbonate monomers or oligomers, cyclic or linear ester oligomers, hydroxy acids, and hydroxy acid esters.