FIG. 1 shows GPCs before and after the depolymerization, and FIG. 2 shows an APCI MS [cyclic M++H2O (18)] which was analyzed in the course of the depolymerization reaction. It was detected that the number of lactic acid units in the cyclic oligomer, which was obtained by use of a calibration curve from the GPCs of FIG. 1, was about three. It was also confirmed that the produced oligomers were cyclic from the fact that degradation products of from a low molecular weight to a relatively high molecular weight were cyclic lactic oligomers in the degradation step as illustrated in the graph of FIG. 2 and the fact that no peak identified as a terminal group was recognized from the 1H-NMR of final degradation products shown in FIG. 3.