PURPOSE: To perform automatic re-grinding of a blade tool the span of the life of which is completed and to prolong and improve an unmanned operation time by mounting a grinding device, automatically grinding according to a preset program, to an NC machine tool body.

CONSTITUTION: A grinding device 16 located in an NC machine tool is driven based on a program previously planned such that automatic re-grinding is carried out when a blade is brought into a state that the cutting life of a blade tool 5 attains a value set by a counter and the like, and the blade tool 5, such as a drill, a reamer, and an end mill, is automatically re-ground. When automatic re-grinding is completed, machining of a work is practicable again, and the grinding device 16 is moved aside to a given position relatively to the blade tool 5 according to a program. Above operation is automatically repeated each time the span of the life of the blade tool 5 is completed.