A method of converting three-dimensional shape data into cell internal data. The method includes an oct-tree division step of dividing external data including boundary data of a target object into rectangular parallelepiped cells having boundary planes orthogonal to each other by oct-tree division. The method further includes a cell classification step of classifying each of the cells into an internal cell positioned inside or outside the target object or a boundary cell including the boundary data, and a cut point determination step of determining cut points of edges of the boundary cell based on the boundary data. The method further includes a boundary surface determination step of connecting cut points to form a polygon, and determining the polygon as the cell internal data when the number of the determined cut points is no fewer than 3 and no more than 12.