PURPOSE: To provide high precision processing stably regardless of thermal displacement by converting the thermal displacement of a machine tool into numerical representation, correcting the coordinates system of a control device using this numerical representation, and by having a corrected coordinates system according to thermal displacement of the machine.

CONSTITUTION: A Y-axis temp. sensor 12 such as thermo-couple is mounted on a spindle 5 supported by a head stock 3 of a vertical machining center, while a Z- axis temp. sensor 13 on a column 2, wherein the output terminals of these sensors 12, 13 are coupled with a temp.-voltage converter 11. A thermal displacement correcting unit 15 receives digital signals given by an analog-digital converter 14 and calculates the amount of correction using an equation provided according to the characteristics of the machine, and the amount of correction obtained is output to correct the coordinates system of NC device. A coordinates system offset unit 16 of the NC device shall correct the coordinates system of the machine upon receiving said amount of correction. Thus high precision processing can be made stably.