PURPOSE: To eliminate variation of input data and a software and to enable continuous automatic operation by setting a given threshold based on a detecting signal for a cutting force right before detection of a first spike signal detected each time cutting is effected one time.

CONSTITUTION: A CPU 11 sets a given threshold based on a detecting signal value for a cutting force right before detection of a first spike signal detected each time cutting is effected one time. A cutting force detector 2 FM-modulates a torque thrust signal, by means of which torque thrust is indicated, to feed it to an antenna 3 and an amplifier 4. An FM modulating signal is detected by separating a high frequency, e.g. noise, by means of a low-pass filter 10 to provide a torque thrust signal. The torque thrust signal is converted into a digital signal by an A/D converted 5 and a result is inputted to the CPU 11. The result value is compared with a given threshold set by means of the CPU 11. When a torque thrust signal value exceeds a given threshold, an alarm signal is outputted through an I/O 16 for an alarm to an alarm device 17, and the alarm device 17 reports the alarm through display or by means of a sound.