PURPOSE: To make it possible to correct the thermal displacement of a machine being operated with excellent accuracy by correcting the thermal displacement by a comparatively simple operational formula on the basis of detected temperature at the specified point of a machine tool, and simultaneously rewriting machine temperature successively into basic temperature for correcting thermal displacement.

CONSTITUTION: Because of the long continuous operation of a numerical control device and change in its environmental temperature, a numerical control device (NC device) causes NC machine tool to produce positional discrepancy due to thermal displacement from program design value. Temperature Ta, Tb at specified points are detected by means of a Y axis temperature sensor 12 mounted on a main spindle 5 and a Z axis temperature sensor 13 mounted on a column 2 to correct the positional discrepancy, and inputted to a temperature, voltage transducer 11 to be coverted into an electric signal, and then inputted to a NC device 16. In the NC device 16, after A/D transducer 17 converts the electric signal into a digital signal, a correcting unit for thermal displacement 18 computes correction value by means of an operational formula to output a signal to correct a coordinate system, and inputs it to a coordinate offset unit 19. The unit 19 corrects the coordinate system of the NC device 16 on the basis of the above input signal.