TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The silicon compound (A) is disiloxanes of formula ((R1)3Si-O-Si(R2)) (I) and disilizanes of formula ((R3)3Si-NH-Si(R4)3) (II). The porous substrate is a porous film with a pore diameter of 2-20 nm provided on a surface of a porous support with a pore diameter of 50-1000 nm.R1-R4 = 1-3C alkyl or 2-3C alkenyl.Preferred Process: The vapor deposition process: is carried out in such a way that the activation energy of the hydrogen gas permeating the gas separator comprising the silica coat formed as a result of the process is no more than 10 kJ/mol at 300-600 degrees C; and comprises a first vapor deposition step of using as the silica source the silicon compound (a) and a second vapor deposition step of using as the silica source a silicon compound (b) that is different from the silicon compound (a). EXAMPLE - alpha -Alumina powder with a mean particle diameter of 1 mu m was kneaded along with water and an organic binder to prepare an extrusion molding paste. The paste was molded using a commercially available extruder, was dried, and was then fired in the atmosphere, to prepare a porous support ( alpha -alumina support) in a tubular shape. A porous film was then formed on the surface of the resulting alpha -alumina support. A boehmite sol was produced through the hydrolysis of aluminum isopropoxide and acid peptization. The alpha -alumina support was dipped for 10 seconds in the boehmite sol, and the support was then taken up at a rate of 1 mm/sec out of the sol, so that the outer peripheral surface of the support was dip-coated with the boehmite sol. This was dried overnight (12 hours) at 60 degrees C and was then fired for 3 hours at 600 degrees C in the atmosphere, giving a gamma -alumina porous film ( gamma -alumina film). This resulted in a porous structure which comprised the gamma -alumina film formed on the outer peripheral surface of the alpha -alumina support. The gamma -alumina film 2 mu m, and the peak pore diameter, as determined by general nitrogen absorption was 4-6 nm. The gamma -alumina film obtained through the firing process at 600 degrees C. The resulting porous structure was set up on a chemical vapor deposition (CVD) device and counter diffusion CVD was brought about using hexamethyl disiloxane (HMDS) as the silica source to form a silica coat on the gamma -alumina film. The counter diffusion CVD was carried out at a reaction temperature of 600 degrees C, a reaction time of 5 minutes, an nitrogen gas feed rate of 200 ml/minute, and an oxygen gas feed rate of 200 ml/minute to obtain the hydrogen gas separator.