PURPOSE: To avoid the influence of metallic particles on an ion implanted substrate, which is caused by the shock of charged particles colliding with the wall surface of a passage for charged beams by making the inner circumferential wall of the passage through which the ion beams pass, divergent forward in cross section.

CONSTITUTION: In order that charged particle beams 3 extracted out of an ion source 2 are shocked onto an objective substrate 4 in which ions are implanted, with specified energy, the mass of ions 3 is measured by an analyzer magnet 6 in an analyzer chamber 5 so as to allow the ions 3 to be introduced into an acceleration pipe 7. The ions 3 are then accelerated so as to be shocked against the substrate 4, and they are also horizontally and vertically scanned. The inner circumferential wall of a passage 13 for the ions 3 is made divergent forward in cross section. By this constitution, the contamination of the substrate 4 can be avoided, which is caused by metallic particles produced by the shock of the charged beams against the inner circumferential wall of the passage 13.