Abstract: In this paper, the design methods of the focus system for micrometerdiameter ion beam are described. The douplasmatorn ion source of cold ca-thode, the double condenser system of the symmetrical three--aperture elec-trostatic lenses, unsymmetrical electrostatic objective lens, the ion beamscanning system of the double electrostatic deflection type without aberrtion,the magnetical analyzer with 12.6°are discussed. The device provides highbrightness of 4.2×~(10)mA/m~2·str at 20Kv, the lifetime of cathode for 1,000hours or more. The ion beams of 2-200 μm diameter are gotten easily.