There is provided a method for easily preparing a polyhydroxycarboxylic acid resin in a one pot reaction by adding 1,3-substituted-1,1,3,3-tetraorganodistanoxan as a polymerization catalyst to a hydroxycarboxylic acid, such as L-lactic acid containing 10 to 50% of water as a starting material, and stirring while heating under a reduced pressure condition or in an organic solvent.