Total sacrectomy is the most efficient surgical treatment for preventing the recurrence of malignant sacral tumors. This treatment requires a reconstructed structure as a replacement for the sacrum, which can support the weight of the upper body. On the other hand, large components cannot be used to reconstruct the structure because of the risk of infection. Therefore, the reconstructed structures of the sacrum must be designed under the severe requirement of the ability of support a heavy weight with minimum structural components. Although several designs of reconstructed structures, in which the lumber vertebrae are connected to the pelvis by metal rods, bars and screws, have been proposed, the size and layout of the instruments have depended on only constraints due to operative procedures, not mechanical considerations. The reliability of the reconstructed structures has been proved empirically, but quantitative evaluations of rigidity and mechanical stress have not been sufficient. In this study, finite-element analyses of two types of reconstruction, which are applied in clinical use, were carried out to obtain stress distribution and total deformation. Advantages and disadvantages of the reconstructed structures were discussed by comparing the results. Furthermore, an improved reconstructed structure was proposed and its mechanical effectiveness was examined by finite-element analysis and a model experiment.