V-CAD data is prepared by dividing external data 12 consisting of boundary data of an object into rectangular parallelepiped cells 13 having boundary planes orthogonal to each other in accordance with octree division and separating the respective divided cells into internal cells 13a positioned on the inner side of the object and boundary cells 13b including a boundary face, and a modeling unit quantity of a prototyping material 7 is changed in accordance with sizes of the internal cell 13a and the boundary cell 13b of a modeling portion. The prototyping material 7 is a resin, lumber powder, a low-fusing-point metal, metal powder, ceramics powder or a mixture of a binder and one of these materials, and its modeling unit quantity is set in such a manner that the modeling unit quantity is smaller than a capacity of a corresponding cell and does not protrude from the boundary plane of the cell. As a result, by using the V-CAD data, a program for rapid prototyping can be simplified, and a model production time can be greatly reduced.