An optically active block polyester copolymer formed of structural units represented by the following general formulas (I) and (II): ##STR1## wherein represents for an asymmetric carbon atom and m and n each represents a natural number in the range of from 300 to 5000 is obtained by subjecting an optically active butyrolactone and a species of lactone other than the optically active butyrolactone to sequential ringopening copolymerization in the presence of a catalyst. This optically active block polyester copolymer possesses a high molecular weight and a high melting point (not less than 130.degree. C.) and forms a biodegradable and hydrolyzable material of high strength capable of tolerating the impacts of actual use.