Abstract: A numerical control (NC) device 10 for a machine tool 1 provided with a measurement probe 2b effects measurements of a work 4 on the machine table 5 after a working process. A measurement program, generated by the NC device by modifying a working program, controls the relative movement of the probe 2b with respect to the workpiece 4 such that the probe 2b moves along the contour of the worked portion of the workpiece 4 with a displacement from the neutral position thereof, wherein the displacement of the probe is sampled at a predetermined sampling period. A working error calculation program 17 calculates the working error from the sampled displacements. If the working error is judged correctable, a outside tolerance range and correction re-working is effected so as to reduce the working error. The measurement/re-working cycles are repeated until the working error is within tolerance.