TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preparation: No general preparation is given. Preferred Composition: The aromatic polyimide film consists of sulfonated aromatic polyimide having polyimide-containing structural unit (20-80 mol%), preferably of formula (III).Ar,X,Ar1=same as defined above;andAr2=divalent organic group which does not have sulfonic-acid substituent.The aromatic polyimide film consists of sulfonated aromatic polyimide having polyimide-containing structural unit (50 mol% or less), more preferably of formula (IV).Ar=same as defined above;andAr3=aromatic ring, trivalent group which does not have sulfonic-acid substituent. EXAMPLE - Ethyl alcohol (430 ml), aqueous solution of caustic soda (250 ml), zinc powder (in g) (84) and 3-nitro diphenyl (75) were added, heated for 5 hours, cooled and filtered. Then, zinc powder was removed and ethyl acetate was washed to obtain azo compound (65). The obtained azo compound (60), ethyl alcohol, (196 ml), aqueous solution of saturated ammonium chloride and zinc powder were added and refluxed for 1 hour. Then, reaction liquid was cooled, filtered and washed, to obtain hydrazo compound (40). The obtained hydrazo compound (30), hydrochloric acid and aqueous solution of sodium hydroxide were added, neutralized and washed, to obtain white powder (15) having melting point of 152degreesC. 2,2'-bisphenyl benzidine (6.72) and fuming sulfuric acid were added, heated at 60degreesC for 2 hours, cooled and poured into methanol (200 ml). Then, the deposited white solid was re-crystallized, to obtain sulfonic acid group-containing compound (8.63) of formula (V). 2,2'-bis(4-sulfo phenyl) benzidine (2.480), triethylamine (1.7 ml), m-cresol (22 ml), 1,4,5,8-naphthalene tetracarboxylic acid dianhydride (1.340) and benzoic acid (0.85) were added and stirred at 180degreesC for 4 hours. The polymerized reaction liquid was then thrown into acetone, to obtain solid having reduced viscosity of 3.8 dl/g. The obtained product was dissolved in m-cresol and flow-spreaded on a glass plate, dried at 120degreesC for 10 hours, to obtain triethylamine salt-type sulfonated polyimide film. The film was then immersed in methanol for 1 day and immediately immersed in sulfuric acid solution for 3 days, and subjected to proton exchange, to obtain sulfonated polyimide proton-type 1,4,5,8-naphthalene tetracarboxylic acid dianhydride-2,2'-bis(4-sulfophenyl)benzidine film. The polyimide film has excellent water resistance, mechanical strength, heat resistance and proton conductivity. The obtained film contains aromatic polyimide compound of formula (VI).