PURPOSE: To faithfully express real things on a computer by discriminating whether a dividing processing of a line segment is for the front side or the rear side, and expressing a graphic by using the line segment whose front side and rear side can be independently divided based on dividing state information.

CONSTITUTION: A line segment two side discriminating part 14 discriminates the front side and rear side of the line segment according to a constant refer ence. A two side line segment graphic model managing part 15 controls an operation to constitute a graphic model while referring to the discriminated result of the line segmesnt two side discriminating part 14. A graphic data storing area 17 stores graphic data being a final aim. An overlap (interruption) between graphic data stored in a data inputting area 16 and graphic data stored in the graphic data storing area 17 is processed, and then, the data of the data inputting area 16 are added to the graphic data storing area 17.