PURPOSE: To detect chipping of edge accurately by extracting a signal composed only of inherent torsional frequency component in the cutter drive system from current or torque signal and when it will exceed over threshold level, producing a pulse and comparing with a rotary signal to produce the chipping signal.

CONSTITUTION: Spindle motor current signal taken out through a shant or CT or torque signal 101 to be detected through a torque detector is detected only of inherent primary torsional vibration component in the cutter drive system through pulse generating circuit 1, and when it will exceed over the threshold level, a pulse signal 102 is produced. Said threshold level is set with correspondence to the load while measuring the magnitude of signal under normal cutting. The pulse signal 102 is checked of synchronism with the cutter rotation signal in a chip signal output circuit 2 and if it is not synchronous, it is decided to be disturbance while if it is synchronous, a chip signal 104 is produced at the time point when several pulse counts have completed to perform such functions as display, M/C stop, etc. by means of a chipping signal detecting circuit 3.