NOVELTY - Diol monomers and dihalide monomers are dissolved in solvent, contacted with a basic compound, and dehydrated. After dehydrating quantity reaches 50% of stoichiometric value, and total content of monomers is 10-100 mol%, 20-100 mol% of diol and dihalide monomers having ionic group are added and dehydration is carried out. The obtained polymer fluid is solid-liquid separated and cast-coated into membranous substance on base material, and solvent is evaporated. The membranous substance is contacted with aqueous acids to obtain an electrolytic membrane.    USE - Manufacture of electrolytic membrane used for electrochemical apparatus e.g. fuel cells, especially using hydrogen and methanol as fuel, water electrolysis apparatus, and chloroalkali electrolyzers. Uses include but are not limited to mobile telephones, camcorders, personal computers, digital audio players, electrical shavers, vacuum cleaners, motor vehicles, electric tools, rail vehicles, robots, passenger cars, and hybrid type electric power supply source together with secondary battery, or capacitor and solar cell.    ADVANTAGE - The method enables economical manufacture of high-quality electrolytic membranes having excellent balance with respect to power generation characteristics under high-temperature and low-humidity conditions, and durability with respect to dry-and-moisture cycle test.    DETAILED DESCRIPTION - An electrolytic membrane is manufactured using a membranous substance consisting of a polymer containing hydrolyzable group and ionic group, obtained by desalination polycondensation of diol monomers and dihalide monomers, as precursor. The sum total content of diol monomer having ionic group and dihalide monomer having ionic group is 0-20 mol%. The diol monomers containing 20-100 mol% of diol monomers having hydrolyzable group, and dihalide monomers are dissolved in solvent, contacted with a basic compound, and heat-dehydrated. After the dehydrating quantity reaches 50% of stoichiometric value, and the sum total content of dihalide monomer and diol monomer is 10-100 mol%, 20-100 mol% of diol monomer and dihalide monomer are added and heat-dehydration is carried out. The obtained polymer fluid is solid-liquid separated directly by centrifugation. The polymer fluid is cast-coated into a membranous substance on a base material, and the solvent is evaporated by heating. The membranous substance is contacted with aqueous acids to obtain an electrolytic membrane.