PURPOSE: To provide a rotating tool abnormality predicting device capable of detecting even the slightest fluctuation of detected data with high accuracy in the initial stage with little wrong action caused by noise, dispensing with the frequent resetting of threshold value.

CONSTITUTION: The cutting force of a rotating tool is detected by a cutting force detector 10, and the detected cutting force is analyzed by a frequency characteristic analyzer 21. Out of this analyzed data, the maximum value of specified frequency band elements or the integrated value of the specified frequency elements is compared with the specified threshold value by a computer 22.