PURPOSE: To improve the efficiency of partial change of elements after division of a shell to elements by generating elements of a moved face based on movement constituting points.

CONSTITUTION: Constituting points p6, p7, p10, and p11 referred to by four elements in a designated area are deleted from constituting points constituting the elements of a shell 30, and elements Y1',...Y9' of a moved face a length (d) distant from the shell are generated based on respective movement constituting points. Elements YS1,...YS12 of the side face are generated based on constituting points p1 to p3, p8, p12,...p9, and p5 in the outer periphery of movement constituting points and constituting points p1 to p3, p8, p12,...p9, and p5 in the outer periphery other than deleted constituting points. Therefore, connection relations of new elements Y1',...Y9' of the movement face and elements YS1,...YS12 of the side face to elements on the shell 30 side connected to elements YS1,...YS12 of the side face are defined, and a moved face 31 and a side face 32 projecting from the original shell 30 are formed.