By external data acquisition means using a computer, external data comprised of boundary data for an object is obtained; by external data input means, the external data are inputted into the computer; by cell dividing means, the external data are divided to rectangular parallelepiped cells that boundary planes intersect perpendicularly; by cell sorting means, the individual cells are sorted into boundary cells that include boundary data and non-boundary cells that do not include boundary data; by space sorting means, respective spaces are assigned to different space numbers where the cells are partitioned according to the boundary data; and by space number compression means, adjacent cells are reassigned to the same space numbers for cells that are not partitioned according to the boundary data.