PURPOSE: To obtain an easy sensing of any abnormality due to wear of a tool or malfunction due to noises, by sampling the amperage to be fed to a motor for drive of the tool, comparing the accumulated value for a certain specified period to that for the model processing, and thereby sensing off any abnormality with the tool.

CONSTITUTION: Selector circuit 32 is changed over to the memory device 33 side to carry out a model machining by a machine tool 10 into a certain intended configuration. The drive amperage MI for the motor 11 to drive the tool, at this time as above, is sampled by an AD converter 22 to make conversion into digital values, and they are accumulated by an integrator 31 for a certain specified period. The accumulated values DVM are inputted in a memory 33 one after another. During an ordinary processing, the selector circuit 32 is changed over to the ratio operator 34 side to calculate at the ratio operator 34 the ratio of the accumulated value DVN in actual processing to the accumulated value DVM for model processing. The ratio obtained RA is placed on a comparison with the setting value ST at a comparator 36, and resulting in any abnormality shall actuate an alarm 26. Thus any abnormality with a tool 13 can be sensed off.