PURPOSE: To improve reliability by detecting the damage of a drilling tool in a way that it is detected that the torque signal voltage of a feed shaft motor exceeds a threshold voltage and a time in which the torque signal voltage exceeds the threshold voltage exceeds a preset time.

CONSTITUTION: A torque signal is fetched from a servo amplifier 15 and the damage of a drilling tool is decided by detecting a fact that the torque signal exceeds a threshold in a time in succession. Since, when a drilling tool is broken, torque of a feed shaft motor 13 is rapidly increased and maintained at abnormal high torque, when the level of the torque signal voltage of the feed shaft motor 13 exceeds the level of a preset threshold voltage in a time in succession, breakage of the drilling tool is detectable.