A low‐energy focused ion beam (FIB) would be a very useful tool for shallow doping with low damage density to the substrate. For a low‐energy FIB, an optical system using a retarding field is advantageous because of a low chromatic aberration coefficient which could not be reduced with a conventional column. Some evaluations were made for an optical column with a retarding electrode for practical use. The secondary electron trajectories within the equipotential space were calculated by the surface‐charge method, and some secondary‐electron images were observed with an experimental column having a retarding electrode.