PURPOSE: To continue an unmanned operation without suspending cutting until the cutting abnormality of a cutting edge is detected by providing an arithmetic process part which performs the arithmetic processing of the measurement data of the cutting edge of cutting time detected elapsedly by a detection means and the normal time data of the cutting edge.

CONSTITUTION: The physical variation of the cutting edge 1 for cutting is detected by a detection means 7. This physical variation of the cutting edge 1 of cutting time detected elapsedly is input as a measurement data and the data of the normal time of the cutting edge 1 as the normal time data to an arithmetic processing part 90 respectively, the measurement data and normal time data are subjected to arithmetic processing based on a specific program and the abnormal cutting by the cutting edge 1 is monitored.