The nonproportionality between the ion current and the electron current in a Bayard‐Alpert ionization gauge is systematically examined and theoretically treated. The effective pumping speed is so modified as to include the effect of back diffusion of oil vapor from the diffusion pump and the contribution of ionization pumping. The theory is experimentally verified in some respects to deduce a new criterion for the maximum attainable vacuum with the diffusion pump. Oil and mercury diffusion pumps are compared to deduce the superiority of mercury pumps for obtaining ``ultra‐high'' vacuum. The best vacuum (in normal working conditions) is 10−9∼10−8mm Hg with oil and 10−11mm Hg with mercury.