A cellulose acetate solution is filtrated with a filter medium formed of cellulose fiber. Hydroxyl groups on the filter medium are substituted to other functional groups. An acidic material is added to the cellulose acetate solution, and the filtration is preformed. In order to estimate the hydrogen ion concentration of the organic solvent, a sample solvent of thereof is mixed with water so as to transport hydrogen ions from the sample solvent to the water of a predetermined volume. Then the water is separated from the sample solvent, and the pH value of the water is measured with a pH meter. The result of the measurement is regarded as the pH value of the organic solvent.