To detect abnormalities of a machining cutter by starting a warning means when a discriminating means judges that a value corresponds to either peak value upper and lower discriminated values or a cutter break discriminated value.

Either command that the rotation of an electric motor 4 for driving a machining cutter 9 is stopped or that an annunciation means 3 is started is performed according to the wear level of the machining cutter 9 output from a discriminating means 11. The judging values of the absolute value measurement and the relative value measurement and the machining load rise judging value are input in this discriminating means 11, abnormalities such as break of the machining cutter 9, the poor cutting edge, overload, abrasion, etc., are discriminated on the basis of the judging values of the absolute value measurement and the relative value measurement, and the using limit of the machining cutter 9 is discriminated on the basis of the machining load rise judging value.