A computer acquires maximum lengths of a structure as measured along X-axis, Y-axis, and Z-axis directions, and acquires numbers of divisions of line segments which extend along the X-axis, Y-axis, and Z-axis directions, respectively, and have the maximum lengths, respectively. The computer then calculates a basic shape, which is the shape of a rectangular parallelepiped divided into a three-dimensional grid, on the basis of the maximum lengths and the numbers of divisions. The computer creates a basic frame of the structure from the basic shape by deleting a selected line, and adding a line between two selected grid points. The computer calculates structure data, which are three-dimensional data representing the structure, by arranging frame members, which are represented by three-dimensional frame member data, along lines between the grid points of the basic frame. The computer then outputs a drawing of the structure from the structure data.