An object of the present invention is to provide an aliphatic polyester which is produced as a high-molecular-weight one without using a chain extender such as an isocyanate compound or a carbonate compound and is excellent in moldability in injection molding, blow molding, or extrusion molding, thermal stability, and mechanical properties such as tensile properties.The invention relates to an aliphatic polyester comprising an aliphatic diol unit, an aliphatic dicarboxylic acid unit, and a unit of an organic acid having a pKa value of 3.7 or below at 25.degree. C., wherein a content of a carbonate bond contained in the polyester is less than 1 mol %, a content of a urethane bond is less than 0.06 mol %, a content of an aliphatic monooxymonocarboxylic acid unit is less than 0.02 mol %, a content of the phosphorus atom derived from a phosphorus compound selected from among organic phosphinic acids, organic phosphonic acids, and hydrogen phosphate salts is less than 10.sup.-9 mol %, based on the whole monomer units constituting the polyester, and a reduced viscosity (.eta.sp/C) is 1.6 or more.