PURPOSE: To prevent the generation of the breakage of a tool during working by estimation-judging the life of the tool on the basis of the load value of a spindle motor in a working machine.

CONSTITUTION: A main spindle is revolved when a tool is separated from a work, and the initial load value of a spindle, motor is detected and a fixed value is added with the initial load value, and the level SI of the 'threshold value 1' and the level S2 of the 'threshold value 2' are set. In the step working where a tool is advanced intermittently, if the load value of the spindle motor exceeds the level S1 of the 'threshold value 1', the tool is once separated, and working is executed again from an original position. In the next working operation, if the load value of the main spindle motor exceeds the level S2 of the 'threshold value 2', the tool is separated from the work, and working is executed again. When the number N of times of the operation for separating the tool from the work exceeds a set value, it is judged that the life of the tool is exhausted, and the tool is replaced.