To detect tool breaking during working in real time under a machining condition changing in tool, machining condition, and work material quality.

A CPU of CNC device monitors during machining operation at an interval of an unit time Δt for instance 10 milliseconds. In case of drill machining, CPU computes a command rotational quantity of a main spindle per the unit time as a determination reference value B for detecting tool abnormality based on the rotating speed data of the main spindle and so on designated in NC program (a step 52), computes an actual rotational quantity ΔQ of the main spindle per the unit time (a step 55), and determines the occurrence of tool abnormality by comparing a difference between the command rotational quantity B and the actual rotational quantity ΔQ with a threshold value E (a step 57). In taping machining, the determination reference value B is computed as an increment of a rotational quantity of the tap (Pn-Pn-1) per the each unit time based on each rotational position of the tap determined in relation to each position of cutting depth per the unit time.