TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The sulfonic acid group inducing agent is a sulfonated agent, which is a proton acid and its derivative(s) containing sulfur atom. A sulfone group inducing agent is used as additive for solution esterification. The additive is a proton acid and its derivative(s), a metal halide, a fatty amine or an amine salt.    TECHNOLOGY FOCUS - POLYMERS - Preferred Component: Polyarylene has repeating structural unit of formula (A1 or A2) and (B).A=bivalent electron attractive group;B=bivalent electron donor group or bivalent direct bond;Ar=aromatic group having substituent of formula: -SO3Rb;Rb=alkyl group;m,n=0-10;k=1-4;Y=H or sulfonic acid ester group;R1-R8=H, F, alkyl, fluorine substituted alkyl, allyl, aryl or cyano group;W=bivalent electron attractive group or bivalent single bond;T=divalent organic group or single bond;p=0 or more;andArasterisk=aromatic group, which does not have sulfonic acid ester group or sulfonic acid group.Preferred Process: The film formed using varnish composition, is immersed in aqueous solution having pH of 1-4, at 5-90degreesC, and further in aqueous solution having pH of 4-7.Preferred Composition: The proton conduction film contains 0.3-5 meq/g of sulfonic acid group. EXAMPLE - A sulfonated polymer precursor (in g) (5.1) having number average molecular weight of 47600 and weight average molecular weight of 159000, was dissolved in N-methyl-2-pyrrolidone (30 ml). Lithium bromide (1.26) was added to the resulting solution, and uniformly stirred at room temperature. The obtained solution was coated on a polyethylene terephthalate substrate, predried at 80degreesC for 1 hour, and dried at 150degreesC for 1 hour. The cast film was washed twice with sulfuric acid aqueous solution having pH of 1, and further with ion exchange water until pH of wash water became 6 or more. Sulfonic acid ester group contained in the obtained polymer film was converted into sulfonic acid group. The polymer film had sulfonic acid equivalent of 1.9 meq/g, proton conductivity of 0.035 s/cm at 25degreesC and 80% relative humidity, fracture strength of 3.5 GPa, inflection temperature of 171degreesC, storage elastic modulus of 84 MPa and elasticity of 46%. The polyarylene copolymer having sulfonic acid group had weight average molecular weight of 185000 and number average molecular weight of 53200.