TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Components: The phase-separated structure is a sea-island structure with carbon-containing phase as the island and inorganic phase as the sea, or is in the form of a continuous structure.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The skeleton section of the carbon-containing compound is substituted with hydrogen at a joint with the three dimensionally crosslinked silicon-oxygen structure to satisfy the relation of (delta p(power 2) + delta h(power 2))(power 1/2) of at most 7 (MPa)(power 1/2), where delta p and delta h are polarity and hydrogen bond components of three-component solubility parameter. The skeleton section of the carbon-containing compound is a hydrocarbon having a structure of -(CH2)n- (n = 2-20) or -CH2CH2-(C6H4)n-CH2CH2 (n at most 4) or -O-(Si(R1)(R2)O)l-. The carbon-containing compound is bound to the three-dimensionally crosslinked silicon-oxygen structure via at least 2 bonds. The hydrolyzable silyl groups are of formula R4(SiXm(R3)3-m)2, preferably (R3)3-mXmSi-(CH2)n-SiXm(R3)3-m, (R3)3-mXmSi-CH2CH2(C6H4)n-CH2CH2-SiXm(R3)3-m or (R3)3-mXmSi-O-(Si(R1)(R2)O)l-SiXm(R3)3-m.l, n = 2-20;m = 3 or less;R1-R3 = CH3, C2H5 or C6H5;R4 = hydrocarbon compound;X = Cl, OCH3, OC2H5 or OC6H5.Preferred Method: The hydrolyzing/condensing of hydrolyzable silyl group uses water at 5-40 degrees C for at least 2 hours. After the hydrolyzing step, the silyl group is aged at 100-300 degrees C.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The inorganic acid is a heteropoly acid supported by fine particles of metallic oxide. The heteropoly acid can be tungstophosphoric, molybdophosphoric or tungstosilicic acid. The membrane has a three-dimensional crosslinked silicon-oxygen structure covalently bonded with the carbon-containing compound. EXAMPLE - 0.8 g of 1,8-bis(triethoxysilyl)octane dissolved in 1.5 g isopropyl alcohol were mixed with 0.7 g tungstophosphoric acid dissolved in 1.5 g isopropyl alcohol. The mixture was left at 20 degrees C for 15 hours, and heated at 60 degrees C for 10 hours to prepare the transparent membrane. The membrane was washed with water at 60 degrees C and then analyzed by field emission type electron microscopy to produce a Z-contrast microgram. The skeleton section of the carbon-containing material was a saturated hydrocarbon having (delta p(power 2) + delta h(power 2))(power 1/2) of almost 0 (MPa)(power 1/2). The phase-separated structure with the inorganic acid had formed the continuous phase. There was a clear contrast between the phase containing the carbon-containing compound and that containing the inorganic acid. Tests showed that the membrane could be bent, and that it had conductivities of 3.4x10(power -2) s/cm at 60 degrees C and 1.5x10(power -2) s/cm at 140 degrees C.