The present invention includes an external data acquisition step (S1), an external data input step (A), a cell division step (B), a cell classification step (C), a space classification step (D), a simulation step (S3), and an output step (S4). The cell classification step (C) includes a step of further classifying each of the boundary cells (13a) into a first type cell and a second type cell. The first type cell has a cutting point at which an edge line or vertex is cut by the boundary data. The second type cell has a cutting point that lies on a boundary with another cell of different hierarchy, and is larger than the another cell. The cell classification step (C) further includes a step of assigning a material number to each cell vertex.