There were weighed 150 mg of PLLA (Mn=110,000), 150 mg of an immobilized enzyme, Novozym 435, and 300 ml of o-xylene, and then they were put into a small pressure-resistant test tube with a screw cap, in which a magnetic stirrer was put. The tube was purged with argon, and then the solution was stirred at 100° C. for a day to depolymerize PLLA. Next, thereto was added a small amount of chloroform, and then the immobilized lipase which was insoluble was filtrated off. An evaporator was used to concentrate the solvent from the filtrate under reduced pressure to yield oligomers including, as main components, cyclic oligomers, at a conversion ratio of 90%.