A plasma-polymerized thin layer was deposited on the top surface of the skin layer of the asymmetric polysulfone (PSF) membrane to plug ultrafine pores passing through it. Trifluoromethane was used as the monomer gas of plasma polymerization. The steady-state permeation rates for oxygen and nitrogen through the asymmetric PSF membranes with the plasma modification were measured to evaluate the permselectivity and permeability. If ultrafine pores passing through the asymmetric membrane can be plugged by the plasma-polymerized thin layer, the separation factor of oxygen relative to nitrogen can be increased with the permeation rate coefficient of oxygen still kept large. This condition was met when the vaporization time of the cast solution during the preparation of an asymmetric membrane ranged from 10 to 120 s and the plasma polymerization time ranged from 15 to 30 min.