The present invention addresses the problem of providing a process for producing continuously and efficiently an aliphatic polyester having an enhanced molecular weight, wherein a diisocyanate can be uniformly mixed with a molten aliphatic polyester prepolymer having a temperature equal to or higher than the melting point thereof with minimized evaporation of the diisocyanate. To resolve the problem, the present invention provides a process for producing an aliphatic polyester having an enhanced molecular weight, said process including: a step (i) of quantitatively injecting a diisocyanate into a hydroxyl-terminated aliphatic polyester prepolymer which is in the form of a melt having a temperature equal to or higher than the melting point thereof and which has a number-average molecular weight of 5000 or more and contains a succinic acid as at least one of the acid components, with the equivalent amount of the diisocyanate injected corresponding to one tenth to two times the equivalent amount of the hydroxyl groups of the aliphatic polyester prepolymer; a step (ii) of mixing, in a static mixer, the diisocyanate-containing aliphatic polyester prepolymer obtained quantitatively and continuously in the step (i), discharging the obtained mixture from the static mixer, and feeding the mixture into a reaction vessel for thickening; and a step (iii) of reacting the aliphatic polyester prepolymer with the diisocyanate in the reaction vessel.