PURPOSE: To improve the machining size accuracy and shorten the machining time by detecting the load current of a servo motor, calculating the distortion quantity of a tool corresponding to it, and using the calculated value as the correction data.

CONSTITUTION: The load current of a servo motor 1 is detected via a shunt resistor 2. On the other hand, the shift data prepared by a pre-processing section 6 is interpolated by an interpolation processing section 7 and is acceleration/ deceleration-processed by a smoothing processing section 8. The output from this processing section 8 and the tool displacement quantity from a position detector 12 are processed by a position loop processing section 9 which is a tool position feedback control means. Furthermore, the backlash and pitch error are corrected by a correction processing section 10, and the distortion quantity of the machine system is calculated by a distortion correction processing section 11. As a result, machining size accuracy better than before can be obtained even by rough machining only. Accordingly, when the roughness of the finished surface is not required too severely, finish machining can be omitted.