TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The low molecular acid is at least 1 type chosen from low molecular phosphoric acid and low molecular sulfonic acid. The low molecular acid contained in a ratio of 0.01-70 wt.pts with respect to 100 wt.pts of polyarylene having sulfonic acid group.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The low molecular acid is at least 1 type chosen from low molecular phosphoric acid and low molecular sulfonic acid. The low molecular acid contained in a ratio of 0.01-70 wt.pts with respect to 100 wt.pts of polyarylene having sulfonic acid group.    TECHNOLOGY FOCUS - POLYMERS - Preferred Polymer: The polyarylene having sulfonic acid groups has repeating structure unit of formula (A) and repeating structure unit of formula (B).A=bivalent electron attractive group;B=bivalent electron donor group or bivalent direct bond;Ar=aromatic group which has substituent with -SO3H;m,n=0-10;k=1-4;R1-R8=at least 1 type of atom or group chosen from H, fluorine atom, alkyl, fluorine substituted alkyl, allyl, aryl and cyano group;W=bivalent electron-attractive group or bivalent single bond;T=single bond or divalent organic group;andp=0 or positive integer. EXAMPLE - Polyarylene copolymer (polyAB-SO3neo-Pe) having sulfonic acid group with number average molecular weight of 53200, weight average molecular weight of 185000, and sulfonic acid equivalence of 1.9 meq/g, was prepared using 4-(4-(2,5-dichloro benzoyl) phenoxy)benzene sulfonic acid neopentyl (A-SO3neo-Pe), BCPAF oligomer of formula (I), phosphorus phenyl chloride compound (PPh32Cl2), triphenyl phosphorus compound (PPh3), zinc dust and N-methyl-2-pyrrolidone. The obtained polyarylene copolymer (in g) (60) was added to gamma-butyrolactone (442) and dissolved. Ortho-ethyl phenyl phosphoric acid (6) was added and mixed for 20 minutes. Uniform dispersion was performed, and the resulting solution was cast on a polyethylene terephthalate film. The resulting cast was dried at 80degreesC for 30 minutes and 120degreesC for 60 minutes, and uniform solid electrolyte film with thickness of 40 microns was obtained. The obtained film had proton conductivity of 0.31 S/cm at 100degreesC, 0.29 S/cm at 120degreesC, and 0.25 S/cm at 150degreesC.