To easily and properly set a tolerable range of a comparison value between a current value and a late past average value concerning an index representing a load added to a tool.

With a machining cycle count index i=O, machining is executed, while the load is detected at a prescribed periodic cycle, and the index H(i) (area S for the load value, maximum inclination absolute value G between two points when the load is lowered, etc.) are formed for n-cycles (S1-5). Comparison values between the respective index for n-cycles for the late past and the index value for the current cycle are formed. For the first-time comparison value, setting of the initial value for a threshold value can be used (S6-10). When any of the comparison values deviates from the tolerable range, the threshold value is updated to expand the tolerable range, and updated history is stored (S11-12). As a damage abnormality is visually checked, a threshold value update stopping command is inputted, and machining is stopped after completion of the cycle. An updated value for the newest threshold value is collected to be a set threshold value (S13-15).