This paper describes a parametric-free optimization method for designing the smooth optimal free-form of automotive shell structures. A stiffness design problem is formulated as distributed-parameter shape optimization problem, and the sensitivity function is derived under the assumptions that the shell is varied in the normal direction to the surface and the thickness is constant. With this method, optimal global and local curvature distribution involving beads can be created without any shape parameterization. The validity and practical utility were verified through several design examples. It was also confirmed the obtained structures became ideal membrane-carrying structures.