The CVD film of methoxysilane derived from diphenyldimethoxysilane(DPDMS) was formed on the outer surface of a porous ${alpha}-alumina$ support tube coated(via dipping-drying-calcining) with a boehmite sol(0.3 mol-Al/L). The resulting silica membrane($500^{circ}C,;P_{fe};=;130;Pa$) showed a permeance of $5.18{ imes}10^{-8};mol;{cdot};m^{-2}{cdot}s^{-1}{cdot}Pa^{-1};for;CO_2$ and a permselectivity of 13.35 for $CO_2/N_2;at;30^{circ}C$.