To predict a cutting loss in a drill by arranging a cutting loss prediction determining part outputting a warning signal when a main spindle torque variation function fluctuates beyond the predetermined value to a torque variation model function.

A cutting loss prediction determining part 6 compares a torque variation function TF, which is outputted from a main spindle torque variation function generating part 9 for each boring by means of a drill 13, with a torque variation model function TF1, in which a normal zone NZ is set, in a function memory 7, and then, it is determined whether the torque variation function TF is within the normal zone NZ of the torque variation model function TF1 or not, that is, whether cutting resistance just after a start of boring by means of the drill 13 is within a drill cutting loss danger free zone or not. When the torque variation function TF exceeds the normal zone NZ of the torque variation model function TF1, a cutting loss warning signal WS is immediately outputted to a main control part 2.