The methanol decomposition to carbon monoxide and hydrogen is catalyzed at 200 degrees C over nickel supported on porous glasses of which major pore diameters are 4, 19, and 45 nm. The catalyst in which the crystallite size of nickel is close to the pore diameter of 19 nm is the most active while in other samples the crystallite size is significantly different from the pore diameter, suggesting that the reaction over nickel particles is enhanced in the pore of which size is close to that of the particles.