Abstract: In the oxidation-polycondensation of a monohydric phenol in a reaction vessel equipped with a reflux condenser, in the presence of a complex catalyst comprising a copper compound and an amine, while passing an oxygen-containing gas thereinto, an improvement is provided wherein the temperature of the oxygen-containing gas to be discharged through the reflux condenser is adjusted to a range of 10.degree.-25.degree. C. to adequately maintain the balance of an amine and water during the course of the reaction. According to this improvement, the reaction advances well although a small amount of the amine escapes outside the reaction system along with a part of water resulting from the reaction. Further, the polymerization reaction can be repeated in a very stabilized state without any fluctuation of reaction time unlike the conventional method wherein the discharged gas through the reflux condenser is deeply cooled down to a temperature lower than 0.degree.C.