A process for producing an aliphatic polyester includes the steps of: heating a cyclic ester at a melting point or higher to transform the cyclic ester into molten state and convert a moisture contained in the cyclic ester into free carboxylic acid; measuring an amount of the free carboxylic acid in the molten cyclic ester; feeding a proton-source compound in an amount determined based on the measured amount of the free carboxylic acid into the molten cyclic ester; and subjecting the cyclic ester to ring-opening polymerization. The process provides an aliphatic polyester with a desired molecular weight which can be set easily and accurately. An apparatus for the process is also provided.