To accurately estimate the relative displacement between a tool cutting edge and a work by grasping the temperature distribution of the whole main spindle system.

The temperature of an outer race of a bearing 10 for supporting the tip of a main spindle 1 is measured by a first sensor 21. The temperature of the outer periphery of a stator 15b in a motor 15 is measured by a second sensor 22. A third sensor 23 is provided on a main spindle head 2 at the position separated from the first sensor 21 and the second sensor 22, and the temperature of the intermediate part to be heated by thermal transfer in rotating of the main spindle is measured. A fourth sensor 24 for measuring the reference temperature is provided on a bed not to be heated with the rotation of the main spindle. Outputs of respective sensors are input into an NC device 26 through an analog-to-digital converter 25, and the relative displacement between a tool 18 at the tip of the main spindle and a work is calculated by the calculating means on the basis of the measured temperatures of respective sensors.