PURPOSE: To surely detect abnormality of a tool by dividing fequencies upon cutting process into low and high frequency distributing ranges of the tool of normal state, and by comparing these frequency components with each others.
CONSTITUTION: Vibration of a tool is detected as an electrical signal by a detector 1, and the electrical signal is passed through a band-pass filter 2 in which flextion due to the vibration generated through a drive shaft or the like is eliminated. The output signal from the band-pass filter 2 is delivered to an arithmetic unit 3 in which autocorelating process is performed for obtaining a vibration frequency distribution. The output of the band-pass filter 6 is passed through a low-pass filter 4 and a high-pass filter 5 so that the output is filtered at a suitable frequency level. As the result, that is, after the ratio between vibration component values in both vibration distributing ranges is obtained by a comparator 6, whether the tool is normal or abnormal is determined in view of the value of the ratio.