TECHNOLOGY FOCUS - POLYMERS - Preferred Component: The principal chain of proton conductor is carbon-carbon bond. The proton conductor has mesogen, crosslinked structure having silicon-oxygen bond and crosslinked structure having carbon-oxygen bond or carbon-carbon bond. The proton conductor has repeating unit of formula (I-III).A=same as P;B=same as defined above;L11,L12=same as L2;R12=same as R3;R13=same as R41;m=1, 2 or 3;n=0, 1 or 2;andx-z=1 or more.Preferred Properties: The proton conductor has ionic conductivity of 0.01 S/cm or more, methanol diffusion coefficient of 5x10-7 cm2/second or less, peel strength of 40 MPa or more, and weight change rate of 1% or less after immersing in 90 wt.% methanol solution for 24 hours.Preferred Process: Polyether and polysiloxane are cross-linked and molded in the shape of film, to obtain filmy proton conductor. EXAMPLE - Acetyl compound of formula (A-1) (in mg) (338), oxetane derivative of formula (K-15) (521) and silicon derivative of formula (B-1) (18) were dissolved in ortho xylene (1 ml). Azobisisobutyronitrile (11) was added to the resulting solution and stirred at 80degreesC for 1 hour. Hydroquinone (10) was added to the reaction liquid and reaction was terminated. Lot of hexane was added to the reaction liquid, and polymer was settled. The obtained polymer was dried except supernatant liquid. Ortho xylene (0.4 ml), isopropanol (1 ml) and butoxy ethanol (0.2 ml) were added to the dried polymer (207) and dissolved. 30% hydrogen peroxide solution (0.29 ml) was added and reacted at 65degreesC for 5 hours. The obtained brown solution was filtered and poured on polyimide film. solvent was evaporated at room temperature for 12 hours. The film was dried at 60degreesC for 4 hours. The coated article solidified in the shape of film was peeled from the polyimide film, washed with ion exchange water, and dried. Thus, opaque and fine brown proton conductor was obtained. The proton conductor had thickness of 113 microns, tensile strength of 48 MPa measured according to JISK-7127, proton conductivity of 2.3x10-2 S/cm at 25degreesC and 95% relative humidity, methanol solubility of 0.6%, methanol diffusibility of 46.4x10-8 cm2/second and performance index of 5.2. Fuel cell equipped with electrode-membrane assembly formed using the proton conductor had stable voltage characteristics.