TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Method: The solution is subjected to casting in a casting vessel containing an outer frame and a sheet.The outer frame and the sheet are separated from each other.The proton conductor is removed from the outer frame.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The acidic group-containing solid polymer is either a polymer containing an acidic group selected from a sulfonic acid group, a phosphoric acid group, or a phosphonic acid group (preferably a perfluorosulfonic acid polymer); or an acidic group-containing hydrocarbon-based polymer.Both the outer frame and the sheet are made of a fluorine-containing polymer material (preferably polytetrafluoroethylene).    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The ionic liquid is a substance in which an ionic bond is formed by a nitrogen-containing organic cation (preferably pyridinium, pyrazolium or pyrrolidinium salt cation, or an aliphatic cation) and an anion (preferably a fluorine-containing organic (preferably trifluoromethanesulfonate anion, bis(trifluoromethylsulfonyl)imide anion, or trifluoroboran), a nitrate anion, a phosphate anion or a sulfate anion).The organic solvent contains water (not more than 20 wt.%), is a liquid having a boiling point of not less than, and comprises propanol or butanol, when the polymer is perfluorosulfonic acid-based polymer; or N-methyl-pyrrolidone, dimethyl-formamide, dimethylacetamide or dimethylsulfoxide, when the polymer is acidic group-containing hydrocarbon-based polymer. EXAMPLE - A proton conductor was obtained as follows. Naflon(TM) (perfluorosulfonic acid polymer) was dissolved in a solvent containing (wt.%) water (10) and propanol (90). (1-Butyl, 3-methylimidazolinium bis(trifluoromethyl)sulfonyl)imide (EMI-TFSI) was then added to the solution at a ratio of 40 wt.% with respect to the weight of Naflon(TM), and the mixture was agitated for 10 minutes to obtain casting liquid. The casting liquid was then subjected to casting in a vessel that was heated at 40 degreesC for 5 hours in a furnace, followed by heating at 150 degreesC for 1 hour to remove the solvent. The proton conductor membrane obtained was then removed. The membrane had a width and length of 30 mm, and a thickness of 50 microns. A comparative proton conductor was obtained as follows. Naflon(TM) (having a width and length of 30 mm, and a thickness of 50 microns) was immersed at 40 degreesC for 24 hours in EMI-TFSI (20 ml) to impregnate the Naflon(TM). The Naflon(TM) was then taken out, excessive EMI-TFSI was wiped out, the weight of the sample was measured, and was found to contain 40 wt.% of the ENI-TFSI, with respect to the initial dry weight. The proton conductivity at 140 degreesC (S/cm) of the test/comparative proton conductors was found to be: 5.7x10 power -3 / 5.6x10 power -3. The retention ratio of the ionic liquid in the proton conductor was also determined by measuring the weight difference of the conductor after immersion in pure water (50 ml), followed by agitation at room temperature for 24 hours, and drying at 1000 degreesC for 6 hours. The retention ratio (%) of the test/comparative proton conductors was found to be 100/5.