PURPOSE: To detect the breakage of the cutting edge of a drill by monitoring the idle cutting time until the drill touches a workpiece by means of an AE sensor.

CONSTITUTION: The AE detecting means 3 directly connected to a workpiece pressing head, an amplifying means 13a to amplify the AE signal obtained by the AE means to a proper level, and a band pass filter 13b for taking out required frequency composition in the range of about 100KHz to 500KHz from the output of the amplifying means are provided. Envelopes are detected by an envelope detection circuit 13c from the output of the band pass filter to compare the output of the envelope detection circuit 13c with the voltage set by a voltage setting device 13d beforehand by means of a comparation circuit 13e. In the next step, idle cutting time is detected by an idle cutting time detecting means 13f from the time lag from the start of drilling to the output of the comparation circuit, and the idle cutting time during the first machining time after drill changing is stored by a reference storage means as a reference value. The idle cutting time during each drilling is compared with the reference value, and if it is longer than the reference value, it is judged as abnormal by an abnormality judging means 13f.