TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The composite electrolyte film contains 2-20 mass% of modified silica.Preferred Property: The modified silica has particle size of 2-10 nm.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Group: The cation exchange group of the polymer is chosen from sulfonic acid group, carboxyl group, phosphoric acid group, imide group, sulfonimide group and sulfonamide group.    TECHNOLOGY FOCUS - POLYMERS - Preferred Group: The polymer has sulfonic acid group as cation exchange group at the terminal of side chain. The polymer further contains a fluorinated polymer having number of fluorine atom of 90% or more in the number of fluorine atoms bonded with carbon atom of principal chain and side chain and number of hydrogen atoms. The polymer has a sulfonate as cation exchange group at the terminal of side chain. EXAMPLE - Sodium salt of dioctyl sulfo succinate (in g) (9.522) and n-hexanol (1.553) were supplied to a reactor and stirred. The sodium salt of dioctyl sulfo succinate was dissolved in n-hexanol. Subsequently, cyclohexane (100) was further supplied to the reactor. A Nafion film Nafion 115 fixed to a membrane kit was introduced into the solution in the reactor. 2 ml of aqueous ammonia was further supplied into the reactor and a white deposit was produced. Subsequently, 3-propyl trimethoxy mercapto silane (2.358) was gradually supplied to the reactor and the reaction was advanced for 96 hours. Sequential implementation of processing of ethanol washing, 1 hour vacuum drying at 100 degrees C, washing with acetone and 100 degrees C 1 hour vacuum drying was performed with respect to the processed Nafion film. The washed and dried Nafion film was dipped in 1M sulfuric acid aqueous solution, then heated at 100 degrees C for 1 hour. The Nafion film was then washed by distilled water and a composite electrolyte film was obtained by vacuum drying at 100 degrees C for 1 hour. The Nafion film was comprised of a high fluorinated polymer. The composite electrolyte film had improved ionic conductivity.