When creating other tool path from two tool paths for cutting a work accurately and efficiently, the data on the first tool path is composed of a position vector and a direction vector of a tool for use when the work is cut while moving the tool. The data on the second tool path is composed of a position vector and a direction vector of the tool for use when the work, which has been cut according to the first tool path, is further cut. A computer uses the first tool path data and second tool path data to calculate a position vector and a direction vector of a medium tool path that is between the first tool path and second tool path.