It can be expected that a microwave acts as an accelerator of the mobility of polar functional groups such as hydroxyl groups in polymeric membranes, and then gas permeability improves because of the increasing free volume and/or temperature of the membranes. The permeability coefficients of CO2 for a cellulose acetate (CA) membrane were measured at 25degreesC using a hand-made apparatus which can irradiate a microwave (2,450 MHz) to the membrane. Both the permeability and diffusion coefficients of CO2 for CA membranes are increased by irradiation of the microwave, and degree of enhancement increased with power of the microwave.