The present invention relates to a method for producing an aliphatic polyester having an increased molecular weight comprising the steps of: (i) introducing into a mixing tank equipped with an inlet for injecting a diisocyanate an aliphatic polyester prepolymer which has a number average molecular weight of 5000 or higher and has terminal hydroxyl groups and in which at least one acid component is a succinic acid compound, wherein the aliphatic polyester prepolymer is introduced above the position of the inlet and wherein the aliphatic polyester prepolymer is melted at a temperature not lower than a melting point of the prepolymer and is in a liquid state, (ii) injecting a diisocyanate into the aliphatic polyester prepolymer, wherein the amount of diisocyanate is equivalent to between one tenth and two times the amount of the hydroxyl groups in the aliphatic polyester prepolymer; and (iii) reacting the aliphatic polyester prepolymer with the diisocyanate.