The NaCaA zeolite was modified by a method of chemical vapor deposition of silicon alkoxide; the resulting silica overlayer on the external surface enclosed the pore-opening size precisely. On the basis of the measurements of rate of adsorption and chromatographic response, it was found that the rate of adsorption of lower olefins (C2H4, C3H6, and 1-C4H8) was suppressed by the modification with the equilibrium adsorption amount unchanged; the extent of suppression depended upon the length of the olefins and the amount of silica deposited. Lower olefins thus can be separated by choosing the extent of silica deposited.