To detect accurately and reduce the abnormal conditions of cutting tools, like piling of cutting chips, frictional consumption, frequent breaking of tools by providing a means for detecting abnormal conditions of the system based on the load detected by a means for detecting the load magnitude.

To keep a main shaft 10 at the stopped condition, it is necessary to generate the rotational force at a motor main shaft 14 to compensate for the reaction. Namely, the mortor shaft undergoes load during stoppage of the motor shaft 14. This load increases as the cutting trash and the frictional consumption increases, and decreases sharply when the cutter tools are broken. Thereby, an electric circuit 30 detect the abnormal condition of the system by detecting the load magnitude.