To avoid a glow discharge during the use of a conventional gas ionization chamber, there is no alternative but to increase a gas pressure. Therefore, while a conventional gas ionization chamber is used, an ion current cannot be increased by raising a gas introduction pressure. An object of the present invention is to increase the ion current by raising the gas pressure and prevent an ion beam from being scattered by an ionization gas. The gas is supplied from a structure maintained at a ground potential to prevent the application of a high voltage to the vicinity of an ionization gas introduction port at which the gas pressure is relatively high. Further, the ionization gas existing in a region through which the ion beam passes is preferentially reduced by performing differential evacuation from a lens opening in a lens electrode that forms an acceleration/focusing lens.