A cyclic ester, for example 3,6-bis[2-(methylthio)ethyl]-1,4-dioxane-2,5-dione, is mixed, in an organic solvent, with an alkyl aluminum compound represented by a formula [I]: Rn Al X3-n, for example trimethyl aluminum, triethyl aluminum, tri-isobutyl aluminum, tri-n-hexyl aluminum, tri-n-butyl aluminum, tri-n-octyl aluminum, diethyl aluminum chloride, ethyl aluminum sesquichloride, ethyl aluminum dichloride, or diisobutyl aluminum hydride. The resulting mixture is then mixed with at least one ring-opening polymerization catalyst selected from the group consisting of a dialkyl zinc and a dialkyl magnesium to allow ring-opening polymerization of the cyclic ester, thereby obtaining a polyester.