In a basic investigation of the processing of emission gas containing volatile organic compounds by vacuum pressure swing adsorption without purge gas, ethyl acetate and methyl ethyl ketone were employed as volatile organic compounds and mesoporous activated carbon and microporous activated carbon as adsorbents, and the effects of feed gas concentration on the lean gas concentration and of the column diameter on the time required to reach the cyclic steady state were measured. Feed gas concentration was found to have little influence on lean gas concentration, which remained constant after the cyclic steady state had been reached. This is attributed to the variation in the average adsorption ratio of the absorbent in proportion to the feed gas concentration. This phenomenon was more marked with mesoporous activated carbon than microporous activated carbon The effect of column diameter as evaluated from the mass-transfer coefficient in the column was found to be greater in the desorption step than the adsorption step, indicating the existence of an optimum column diameter.