PURPOSE: To permit cutting with high accuracy by a construction wherein a sensor is provided in a bit holding bore of an end mill, etc. to monitor a slight axial movement of the bit during cutting, thereby to send a signal indicative of a correction value to a numerical controller.

CONSTITUTION: A sensor 16 is provided in a bit holding bore 7 in which a bit 8 is inserted. If the bit 8 is slightly shifted in the axial direction A during machining, a sensing rod 18 is vertically moved through a slide pin 19, the movement of the rod being detected by the sensor 16. The detection signal is taken out to the exterior through wirings 24, 37 so as to be surely and positively transmitted to a numerical controller which functions to control a machining tool 5.