To accurately detect a tool anomaly and instruct replacement.

In S1/S2, disturbance torque ys and yz acting on a main spindle motor and a Z-axis motor are estimated and they are stored in a data table. Until a time t0 passes from a starting point of time, S1 is repeated and pieces of data of ys and yz are gathered. The time t0 is slightly longer than times T1, T2 and T3. In S3, average values ysav1 and yzav1 (present situation estimates) from the present to the time T1 before are determined in regard to ys and yz. In S4, average values ysav23 and yzav23 from the present to between the time T2 before and the time T3 before are determined in regard to ys and yz. In S5, a movement fluctuation threshold is determined. In S6, the present situation estimates are compared with the threshold. If there is deviation from the threshold, it is determined that there is a tool anomaly, the procedure proceeds to S7, and a tool replacement signal is generated after finishing a machining cycle. A procedure of determining upper and lower limit bandwidths as the movement fluctuation threshold on the basis of an average waveform pattern of several prior machining cycles can be adopted also.