PURPOSE: To provide a technical means in which the processing dimension of a work and secular change of a machine tool to thermal deformation are measured by a NC device for controlling the machine tool, without setting a detection head such as a touch probe.

CONSTITUTION: A detector body 13 is attached to a turret tool rest 5, skip orders are given for making the detector body 13 approach a work 15 and main spindle side members 3 of a machine tool at low speed, in such a condition as restricting the output torque of a feed motor 8 of a turret to the minimum torque which is necessary to move the turret tool rest 5 with no load, so that the approaching motion may be skipped when any increase in the positional deviation c, preseuted by a difference signal between the position order a given to a servo control device 21 of the feed motor 8 and positional feedback signal b, is detected during the approaching motion. Then, the dimension of the work 15 and change in relative positional relation between the turret tool rest 5 and the main spindle side members 3, are calculated from the positional coordinated of a NC device 22 time of the positional order a and the positional deviation c. etc., at time of the skip.