PURPOSE:To economically obtain in a perfectly safe manner the titled copolymer useful as controlled release substrate for drugs, by heating under reduced pressure a mixture of glycolic acid and lactic acid in a non-catalytic state to effect dehydrating polycondensation. CONSTITUTION:The objective copolymer with a molecular weight falling be tween 4,000 - 20,000 can be obtained by heating under reduced pressure a mixture of glycolic acid and lactic acid in a non-catalytic state to effect dehy drating polycondensation.