Total sacrectomy is the most efficient surgical treatment to prevent recurrence of malignant sacral tumors. It is required a reconstructed structure replaced for the sacrum which can support large weight of upper body in this treatment. On the other hand, large components are not allowed to reconstruct the structure because of an infection risk. Therefore, the reconstructed structured of the sacrum have to be designed under the severe requirement that is necessary to hold large weight by using minimum structure components. Although several designs of the reconstructed structure in which lumber vertebrae connected with the pelvis by using metal rods, bars and screws have been proposed, size and layout of the instruments have mainly depended on only constraint due to operative procedure but not mechanical considerations. Reliability of the reconstructed structures has been ensured empirically, but quantitative evaluations of rigidity and mechanical stress have not been sufficient. In this study, finite-element analyses of two types of the reconstruction which were available in clinical use were carried out to obtain stress distribution and total deformation. Advantages and disadvantages of the reconstructing structures were discussed comparing the results. Furthermore, an improved reconstructing structure was proposed and its mechanical effectiveness was examined by finite-element analysis and model experiment.