To easily forecast the breakage of a drill, by outputting the forecast of the breakage of the drill when one of the cutting torque and the cutting thrust of the drill necessary for machining a work, is over its threshold value.

The machining by a drill 15 is stopped at a reference time, that is, at a time when one of the cutting thrust and the cutting torque is over its threshold value, the forecast of the breakage of the drill 15 is output, and a cause of breakage judgement part 22 outputs the result of the judgement on the magnitude of a rate of change of the cutting thrust force, and the magnitude of a rate of change of the cutting torque at the reference time stored in a memory, to a display part 30. The display part 30 displays the forecast of the breakage of the drill 15 output from the cause of breakage judgement part 22, and the result of the judgement on the magnitude of the rate of change of the cutting thrust force and the magnitude of the rate of change of the cutting torque stored in the memory.