An organic polymer solution comprising an organic solvent and, dissolved therein,(a) an organic polymer which has an imino-p-phenylene structural unit of the formula ##STR1## as a major repeating unit and is organic solvent-soluble, (b) a protonic acid having a pK.sub.a value of 4.8 or less or a derivative thereof,(c) at least one member selected from the group consisting of an oxidizing agent which does not have oxidizing ability at ordinary temperature but has oxidizing ability at temperatures not lower than 60.degree. C., and an oxidation catalyst.