PURPOSE:To raise evacuation efficiency. CONSTITUTION:A distribution type ion pump(DIP) 23 is disposed on an outer portion of charged particle orbit 13. Means of preventing light irradiation of DIP 23 is taken by eliminating a portion where light passes. And further, an absorber 31 for absorbing light passing to side wall of DIP 23 is disposed. Accordingly, because DIP is disposed at the position of a large gas source due to photodesorption, the evacuation efficiency is raised. Photoelectrons released from the absorber are contained within a cell of DIP, so that ionization of gas molecules is enhanced to raise the evacuation efficiency. In power connection in a low voltage region, the ignition delay is eliminated.