[en] A compact ion microbeam system, which comprises a three-stage acceleration lens and a duoplasmatron-type ion source and produces beam of several hundred kiloelectron-volts, has been developed to form a beam that has a diameter of several micrometers. Previous studies have indicated that a small divergence angle of an incident beam is required to improve the demagnification of the three-stage acceleration lens. A simulation of an ion beam extraction demonstrated that the optimum distance between the anode and the extraction electrode in the ion source could lead to an ion beam being generated with the smallest possible divergence angle. As a result, a hydrogen beam with a 2.6 μm diameter was experimentally produced at 120 keV.