TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The inorganic filler is silica microparticles having particle diameter of 0.01-20 microns.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The inorganic filler is silica microparticles having particle diameter of 0.01-20 microns.    TECHNOLOGY FOCUS - POLYMERS - Preferred Compound: The aromatic hydrocarbon-type polymer compound contains 10-100 mol.% of repeating structural unit of formula (1) and 90-0 mol.% of repeating structural unit of formula (2).R1-R10 = H or -CmH2m+1;m = 1-10;X1-X10 = H or proton acid group;A1-A3 = direct bond, -CH2-, -C(CH3)2-, -C(CF3)2-, -O-, -SO2- or -CO-; andi,j,k and l = 0 or 1.At least one of R1-R10 is -CmH2m+1. At least one of X1-X10 is aromatic polyether ketone which is a proton acid group. The hydrogen atom of the aromatic ring of formula (1) and formula (2) is optionally substituted by -CmH2m+1, proton acid group, chlorine, fluorine or trifluoromethyl group.Preferred Method: The solid polymer electrolyte film is formed by crosslinking aromatic hydrocarbon-type polymer compound.Preferred Property: The solid polymer electrolyte film has surface roughness of 5-1,000 nm. EXAMPLE - 5,5'-carbonyl bis(2-fluoro benzene sodium sulfonate) (sodium-type sulfonic acid group-containing polyether ketone) (in g) (4) and silica Wakogel LC-5H (0.04) with particle diameter of 5 microns, were dispersed in dimethyl sulfoxide (28), and a varnish was obtained. The varnish was cast on a glass plate, heated to 200degreesC in nitrogen atmosphere. Subsequently, dimethyl sulfoxide was removed and a film was obtained. The film was irradiated on both surfaces and subjected to proton exchange after light cross-linking, and a solid polymer electrolyte film was obtained. The film subjected to light cross-linking, was immersed in 2N sulfuric acid for 24 hours, and acid treatment was performed. The film was washed with distilled water and dried at 150degreesC for 2 hours. The obtained solid polymer electrolyte film had surface roughness of 104 nm, proton conductivity of 0.02 S/cm (at 30degreesC) and methanol permeability of 1.4 micro mol/cm2.minute. The solid polymer electrolyte film had excellent proton conductivity, water-resisting property and adhesivity with catalyst electrode.