A crack-free ceramic composite membrane with micropores has been synthesized by the pressurized sol-gel coating technique using the mixed Al2O3-SiO2 sols. The mixed sols were prepared by mixing nanoparticulate SiO2 and boehmite sols. These sols were more stable at lower pH, but very unstable when their copositions were in the range of 50~75mol% of SiO2 at the same pH. The mixed Al2O3-SiO2 membrane prepared from the mixed sol (0.2mol/$ell$ of solid content and pH=2) containing 40mol% of SiO2 had the mean pore radius of 0.80nm and the specific surface area of 280$m^2$/g. The nitrogen permeability through the coated Al2O3-SiO2 layer was 42$ imes$107mol/$m^2$.s.Pa. It was found that the thermal stability of aluminosilicate membranes, even through similar to that of SiO2 membranes, was much improved in comparison with ${gamma}$-alumina membranes.