Finite element method has become a popular practical tool of the assessment of hull strength for structural designers. However, structural optimization using finite element analysis is not yet a usual practice in actual hull design. The obstacles to realize this may be put down to the followings : (1) enormous amount of computation needed in iterative finite element analysis, (2) difficulty in altering structural contour shape in combination with other design parameters with practical constraint, (3) difficulty in automatic mesh generation that robustly produces well-shaped finite element mesh for any altered contour shape, and (4) necessity of efficient optimization algorithm well suited to this kind of problem. The first obstacle may have been cleared thanks to recent rapid progress in computer technology. In order to resolve the remaining problems and to deal with practical structural design, the authors developed a new structural optimization system. This system combines : (1) parametric definition and representation of structural contour shape by the use of CAD system, (2) automatic mesh generation by TRIQUAMESH algorithm, (3) finite element method, and (4) optimization program using genetic algorithm. In this paper, the authors apply this system to actual hull structural design optimization such as : (1) quasi-elliptic opening shape, (2) hatch corner of container ship, (3) horizontal girder of LNG carrier, (4) slot cut-out shape, and (5) hold frame bracket toe of bulk carrier. Through such examples, it is shown that the approach described in this paper can be used efficiently and practically for structural optimization, and that this system is a strong tool of hull structural design.