An electrode member has a plurality of spherical electrode sections wherein the radiuses of the spherical sections are different from each other. The spherical electrode sections are disposed in a state wherein the center points of the respective spheres match each other and the spherical electrode sections are insulated from each other such that voltages can be independently applied thereto. Electron-passing openings for electrons, which move from the center point to the outside of the electrode member, are formed at positions where the spherical electrode sections and a plurality of straight lines radially extending from the center point intersect each other.