TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The acidic catalyst is chosen from nitric acid and hydrochloric acid.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The methane separation membrane is formed, when hydrocarbon group of hydrocarbon group-containing trialkoxysilane is chosen from 2-6C alkyl group and phenyl group. The carbon-dioxide separation membrane is formed when hydrocarbon group of hydrocarbon group-containing trialkoxysilane is methyl group. Methyl trialkoxysilane is added during manufacture of carbon dioxide separation membrane. Preferred Composition: The organic solvent is alcohol chosen from methanol, ethanol and propanol. The tetraalkoxy silane is chosen from tetramethoxysilane and tetraethoxysilane. The hydrocarbon group-containing trialkoxysilane is chosen from ethyltrimethoxysilane, ethyltriethoxysilane, propyltrimethoxysilane, propyltriethoxysilane, butyltrimethoxysilane, butyltriethoxysilane, pentyltrimethoxysilane, pentyltriethoxysilane, hexyltrimethoxysilane, hexyltriethoxysilane, phenyltrimethoxysilane and phenyltriethoxysilane. The metal alkoxide solution contains 0.005-0.1 mol of acidic catalysts, 0.5-10 mol of water and 5-60 mol of organic solvents, with respect to 1 mol of total amount of tetraalkoxy silane and hydrocarbon group-containing trialkoxysilane. The molar ratio of tetraalkoxy silane and hydrocarbon group-containing trialkoxysilane is 1/9-9/1. The methyl trialkoxysilane is chosen from methyl trimethoxysilane and methyl triethoxysilane. During manufacture of carbon dioxide separation membrane, the metal alkoxide solution contains 0.005-0.1 mol of acidic catalysts, 0.5-10 mol of water and 5-60 mol of organic solvents with respect to 1 mol of total amount of tetraalkoxy silane and methyl trialkoxysilane, and the molar ratio of tetraalkoxy silane and methyl trialkoxysilane is 9/1-1/9. Preferred Properties: The methane separation membrane has ratio of transmittance of methane and transmittance of nitrogen of 1-10, and ratio of transmittance of carbon dioxide and transmittance of nitrogen of 1-100. When molar ratio of tetraalkoxy silane and hydrocarbon group-containing trialkoxysilane is 7/3-1/9, the methane separation membrane has maximum ratio of transmittance of methane and transmittance of nitrogen.