PURPOSE: To install ion electrodes in more places, and increase resistance to an eddy current of a pulse electromagnet by dividing metal coatings into plural parts in the circumferential direction while leaving a part.

CONSTITUTION: Metal coatings 3 and 4 are divided into four parts in the circumferential direction of a chamber, and the earth surfaces 4 being the left and right coating surfaces are continued electrically to a flange 1, and the electrode surfaces 3 being the upper and lower coating surfaces are coated so as to be insulated from the flange 1. High voltage is impressed upon electrode terminals 5 extending to the electrode surfaces 3, and (+) voltage is impressed on the upper side, and (-) voltage is impressed on the lower side. When chamber center vicinity electric potential produced by this voltage is higher than electric potential of a beam, residual gas ionized by the beam is attracted to an electrode without staying around the beam. Thereby, it can be circulated stably without being influenced by a beam ion.