Silica composite membranes were prepared for CO removal from products of methanol steam reforming. A porous stainless steel (SUS) support was used to improve the mechanical strength of the membrane. Supports were prepared by coating Ni-powder SiO2 sols with a particle size of 500 and 150 nm in turns on SUS. Silica top layer was coated on the modified support using a colloidal sol with nanoparticles or a polymeric sol. Density of the membrane surface was controlled by repeating dipping-drying or the whole process of dipping-drying-calcination, and the nanostructure of the top layer was controlled by changing the top layer material. As a result of a mixture gas permeation test of the silica composite membrane using H-2(99%)/CO(1%), the CO concentration of 10 000 ppm was reduced to under 81 ppm. The permeation mechanism through the membrane was mainly molecular sieving.