The present invention sets, as a point (P OM ) to be machined, a single machining point on a plurality of tool path rows, selects, as a machining point (P I(i) ) of interest, a machining point in a prescribed range with the point to be machined as the center, calculates the tool orientation at the point to be machined by way of averaging the tool orientation of the selected machining point of interest, corrects data pertaining to the tool orientation of the point to be machined by way of the calculated average tool orientation, acquires the shape data of a workpiece to be machined and the shape data of a ball end mill to be used, performs an interference check for the workpiece and the ball end mill on the basis of the corrected tool orientation data, and generates a new tool path on the basis of data pertaining to the corrected tool orientation when no interference between the workpiece and the ball end mill occurs.