PURPOSE: To prevent occurrence of inferior machining by issuing a signal for changing drills when a difference between a drill position upon detection of a machining resistance during a first drilling stroke and a drill position upon detection of a machining resistance on and after the second drilling stroke exceeds an allowable value.

CONSTITUTION: A first drilling step is carried out (step 150), a position P1 of a drill in a cutting direction upon detection a machining resistance is stored in a memory A (step 170), 1 is added to a counter N (step 200) and then, the next drilling step is carried out (210), and a drill position in the cutting direction upon detection of a machining resistance is stored in a memory B (step 220). Further, an abrasion loss δ caused by the present drilling step is obtained from the difference between the values stored in the memories A, B (step 230), and the cumulated abrasion loss is substituted with δ0=δ0+δ (step 240). If he cumulated abrasion loss δ0 exceeds an allowable value E (step 250), the process is interrupted, and an indication such that the replacement of drills are required is displayed on the screen 15 of a numerical control device 14 (step 260).