PROBLEM TO BE SOLVED: To rapidly and accurately calculate a tool path plane by using a three- dimensional graphics display device.
SOLUTION: In a processing part, polyhedral model information of a workpiece and tool shape information are inputted (S300), and each shape element composing a sweep shape of a reverse tool with respect to each element of a polyhedral model is arranged (S305 and S306). The processing part stores an ID number, color information and geometric information with respect to each arranged shape element in a storing part (S307 and S308), and it determines subdivided point sequence data of a cross sectional line of an uppermost plane while shifting positions of arranged shape elements in x-axis and y-axis directions by predetermined distances by using a hidden- surface removal process of graphics hardware on the basis of the shape elements composing the sweep shape of the reverse tool (S310 and onward). The processing part detects a bending point of the cross sectional line on the basis of the pieces of determined adjacent point sequence data, and it inserts the bending point into the point sequence data of the cross sectional line. The processing part divides a space into accumulation of cubes, and it determines the uppermost plane of the sweep shape by a marching cube method.