A tool compensation method for cutting a surface on a workpiece having a slanted surface. A radius at a final point is determined by a path computing unit using coordinates of the final cutting point and a tool height. Based on these values thus determined, coordinates of a final offset position, which is aligned with a tool center, is also computed. Corrective values in the directions of the X and Y axes are determined using the computed coordinates and coordinates of a starting offset position, to thereby correct the movement of the tool center.