the present invention is a process for producing a polyester from a dicarboxylic acid and a diol , which comprises polycondensing the dicarboxylic acid and the diol under the presence of a distannoxane catalyst . also , the present invention is a process for producing a polyester which comprises melt polycondensing the dicarboxylic acid and the diol under normal pressure in the presence of the distannoxane catalyst , wherein an organic solvent which dose not dissolved any of the dicarboxylic acid , the diol and the polyester produced from the dicarboxylic acid the diol is present and , whereby , two phases are mainly present .