The optimum midship section design of container ship belongs to the nonlinear constrained optimization problem. The determination of scantlings for the midship region is a very crucial matter out of whole structural design process of a ship. This determination may be a process of satisfying a lot of complicated design constraints empirically, rather than optimizing some objective function. However, in order to realize optimum or rational design for midship region with complicated design constraints, it is necessary to model the design problem as strictly as possible and to accomplish more highly numerical optimization. Real-coded micro-Genetic lgorithm (RμGA) is proposed to find the optimum midship section arrangement of an actual hatchcoverless container ship without handling any of penalty functions.