We discussed the evolution of the skeleton of radiolaria (marine plankton) from a viewpoint of optimization process of frame structure. The genus Mirifusus, a group of Mesozoic radiolaria, has a character of detailed and geometric frame between segments. The geometric framework was evolved from combination of pentagon and rhombus to triangle through pentagon. We treated such frameworks as two dimensional rigid frames with periodic boundary condition. We obtained normalized displacement of upper-side under the condition of compress or tortional force for each framework. The results show that the triangle structure is most effective if the 2-D frame is applied both of compress and torsional forces in most cases. Such a structure corresponds to final frame structure of genus Mirifusus during its evolution process.