PURPOSE: To enable the even generation of the beam in a space without the lower of the irradiation efficiency by displacing hole positions in one electrode or more than two of multiple porous type electrodes against hole positions in other electrodes.

CONSTITUTION: The ion 12 generated in a plasma source 11 is accelerated by a porous type electrode 9. The electrode 9 consists of a plasma electrode 13, a negative electrode 14 and a grounding electrode 15, and the ion beam is accelerated by applying the positive accelerating voltage from an accelerating voltage source 16 to the electrode 13 and applying the negative decelerating voltage from a decelerating voltage source 17 to the electrode 14. At this stage, holes 18 of the central part of the electrode, when holes of the electrode 14 are displaced against holes of other two electrodes, the beam 5 is deflected to the opposite direction of the displacement direction of the holes of the electrode 14 with the work of an electric field lens. The beam from the holes 18 can be thereby deflected to the outside. Consequently, the even beam distribution can be obtained.