TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The solvent is a mixture of a good solvent compound which is a good solvent of the solid electrolyte and a poor solvent compound which is a poor solvent of the solid electrolyte. The contact liquid is soluble to the good solvent compound and the poor solvent compound. The weight ratio of the poor solvent compound to the solid electrolyte in the casting membrane at the time of starting the liquid contacting step is 2 to less than 100%. The contact liquid is an aqueous solution containing a compound having a lower b.pt. than the good solvent compound or water; or an aqueous solution containing a good solvent compound which is a good solvent of the solid electrolyte or water. The contact liquid contains a poor solvent compound which is a poor solvent of the solid electrolyte. The elastic modulus of the wet membrane during the peeling process is not less than 6x105 Pa. The wet membrane is contacted with a liquid soluble to the solvent, where the liquid is aqueous solution containing the solvent or water. The solid electrolyte includes a precursor of the solid electrolyte which a proton acceptor. The contact liquid is purified water. The liquid is a mixture of water and the good solvent compound or a mixture of water and the poor solvent compound. The formula weight of anion contained in the proton solution is 40-1000. The good solvent compound contains dimethylsulfoxide, and the poor solvent compound contains 1-5C alcohol. Preferred Method: Part of the solvent is evaporated from the casting membrane before the liquid contacting step. The time for contacting the casting membrane with the contact liquid is determined in accordance with at least one of hardness of the casting membrane and weight of the good solvent compound in the casting membrane both of which change during the liquid contacting step. The time for contacting the wet membrane with the liquid is preferably not greater than 10 minutes. When the evaporating step is performed, the casting membrane forming step, the evaporating step, the liquid contacting step, the peeling step and the drying step are performed as a continuous process. The casting membrane after contact with the purified water contains less good solvent compound as compared to before the contact. The purified water is contacted with the casting membrane at least by spraying the purified water or by coating the purified water onto the casting membrane. At least one of the casting membrane after the contact with the purified water and the wet membrane is contacted with a liquid which soluble to both of the good solvent compound and the poor solvent compound. A sum of time for contacting the casting membrane with the purified water and time for contacting at least one of the casting membrane and the wet membrane with the liquid is not greater than 10 minutes. The method further comprises a proton substitution step for contacting one surface of the wet membrane including the precursor with the proton solution having proton concentration of not less than 0.05 mol/L; and a washing and drying step for washing and drying the wet membrane after the contact with the proton solution. The other surface of the wet membrane is contacted with a low-concentration proton solution having, the proton concentration lower than the proton solution by not less than 0.05 mol/L. The temperatures of the wet membrane at a time of the contact with the proton solution or the low-concentration proton solution, and temperature of the proton solution and the low-concentration proton solution are not less than 30degreesC to not more than glass-transition temperature of the precursor. The proton substitution step and the washing and drying step are performed successively.    TECHNOLOGY FOCUS - POLYMERS - Preferred Compounds: The solid electrolyte is a hydrocarbon polymer, such as aromatic polymer having a sulfonic acid group. The aromatic polymer is a copolymer formed of structural units of formulae (I)-(II) or -(O-Z-O)-(III).X=cation species;Y=SO2;Z=compounds of formulae (Z1)-(Z2);n/(m+n)=0.1-0.5