To prevent the life of a tool from being shortened due to the increment of cutting quantity of the tool on the corner part of an offset route and the excess of tool load at the time of the offset working of contours.

In the case of preparaing a contour offset tool route to be applied to a metal mold by using a tool shape model in a CAM system, the uncut shape 23 of the metal mold is judged from an offset route 22 existing just before an offset route on which a tool shape model 21 tries to advance. Then the length 27 of a circular arc on the model 21 within a range from a rectangular direction to the advancing direction of an offset route 24 to which the model 21 tries to advance up to a direction directing the center 26 of the model 21 to an intesection 25 between the model 21 and the uncut shape 23.