To compensate not only a holding position error of a workpiece and an outline dimension error of a reference plane in a boring process but also the influence of the contraction of the workpiece independently of the condition of contraction.

Four reference marks for encircling a boring position are previously applied to specified positions on the surface of the workpiece and a split ratio is calculated from their specified reference mark position data and predetermined boring position data, at which the sides of a rectangle formed by the reference marks M1, M2, M3, M4 are split by a perpendicular from the specified boring position P. In the boring process, the positions of the reference marks of the workpiece being held are found, split points where the sides of the rectangle formed by the reference marks M11, M12, M13, M14 are split at the split ratio are found from the found reference mark position data, and the position of a crossing P' of two lines connecting the opposed split points is found, so that the boring position data for the specified boring position is rewritten by using the crossing position data.