PURPOSE: To compensate the thermal deformation by providing more than two temperature sensors along single working shaft and obtaining the thermal displacement from a signal from the temperature sensor.

CONSTITUTION: The referential temperature is detected by a referential temperature sensor 24 while the temperature along a working shaft is detected by temperature sensors 20, 22 then the thermal displacement of the working shaft is obtained by a thermal displacement operational unit 48 and a thermal displacement correction signal 118 is fed to a servo motor control circuit 32 to correct the moving command to the servo system. Consequently the thermal displacement can be obtained easily on the basis of the signal detected by each temperature sensor to correct the erroneous position caused by the heat.