TECHNOLOGY FOCUS - POLYMERS - Preferred Film: The polymeric electrolyte film alternately consists of sulfonated aromatic polyimide having structural unit of formula (2), or sulfonated aromatic polyimide having structural unit of formula (2) and formula (3), or sulfonated aromatic polyimide having structural unit of formula (1 and 3). The sulfonic acid or its derivative is contained in 0.5 milli-equivalent or more per gram.Ar3,Ar6 = Ar1, preferably 1,4,5,8-naphthalene tetracarboxylic-acid residue;Ar4 = groups of formula (2-1); andAr7 = divalent group having aromatic group(s) or divalent group which does not have sulfonic acid group as substituent.D2 = D1;R4-R7 = hydrogen and/or 1-2C alkyl group;Ar5 = groups of formulae (2-2) to (2-5);D6 = bond, O,CH2,C(CH3)2,C(CF3)2,SO2,S or CO;R8-R10 = R4;X = same as defined above; andn = 0-2. Preferred Definitions:    Ar1 = 1,4,5,8-naphthalene tetracarboxylic-acid residue.    EXAMPLE - 4,4'-bis(4-amino phenoxy biphenyl-3,3'-disulfonic acid (in g) (5.28) and triethylamine (3.5 ml) were added to m-cresol (40 ml). After complete dissolution of diamine, 1,4,5,8-naphthalene tetracarboxylic-acid anhydride (2.68) and benzoic acid (1.55) were added. The mixture was stirred at 80degreesC for 4 hours and at 180degreesC for 20 hours. The solution was poured into acetone, after cooling to room temperature. The precipitated solid was filtered, dried and dissolved in m-cresol. The salivation was performed on a glass plate and a flexible sulfonic-acid triethylamine salt type film was obtained after drying at 120degreesC for 10 hours. The film was immersed in methanol at 60degreesC for 1 hour and then into 1 N hydrochloric acid solution for 5 hours. The film was washed, after proton exchange. The film was vacuum dried at 150degreesC for 10 hours, and the alkyl group of triethylamine was completely eliminated which indicated that proton exchange was completely performed. The film was found to have excellent water resistance at 80degreesC for an immersion time of 250 hours, and excellent proton conductivity at 80degreesC.