A side wall 35 that extends in the axial direction enclosing the plasma in such a way that expansion of plasma to the sides is prevented at the back surface of a skimmer cone 33 and a small collision chamber 36, which is positioned at the back side of this side wall 35 and is defined by a flat part 56 of a first electrode 53 having an opening 57 through which the ion beam can pass. By means of this small collision chamber 36, the pressure inside the chamber rises without introducing additional gas; therefore, argon ions are neutralized by collision and recombination between the ions and electrons and the ion density of the plasma is reduced. Thus, the beam diameter during ion extraction and transport is maintained relatively small.