The implicit modeling is an important technique in modeling of informal objects. However, the automatic modeling algorithms with the implicit skeletal elements are not very advanced. In this paper, we propose a new skeletal element, Metacube and an automatic modeling technique for informal objects represented by volumetric data. Metacube has the merits of metaball and enhances the modeling ability of informal objects containing the planar surfaces. For 256 ? 256 ? 256 size of input volume, the proposed method takes a short generation time within ten minutes on an SGI indigo2 workstation.