Some methods are known as the methods for producing polyhydroxycarboxylic acids of a high molecular weight by the use of Lactides; for example, a method which comprises polymerizing, in the presence of polyvalent heavy metal oxide such as lead oxide, a lactide which is obtained by heating lactic acid (U.S. Pat. No. 2,703,316), a method for producing copolymers of lactic acid and glycolic acid having an average molecular weight of 72,000 by copolymerizing, in the presence of stannous octanoate, a mixture of lactide and glycolide respectively obtained by dehydration condensation of lactic acids and glycolic acid (Japanese Published Examined Patent Application No. 14688/81), and a method for producing polylactic acids having a molecular weight of 100,000 by polymerizing DL-lactide in the presence of stannous octanoate (Japanese Published Unexamined Patent Application No. 69553/90). However, these methods, in which Lactides of high purity are necessary, require purification steps of crude lactides such as recrystallization and distillation, and thus have various problems for industrial utilization due to a number of complicated.