PURPOSE: To permit to detect the degree of defect or breakage as well as the progressing condition of abrasion of the cutting tool by integrating supersonic signals exceeding a predetermined threshold value and detecting the damage of the cutting tool based on an integrated value.

CONSTITUTION: The device is equipped with a DC amplifier 11 and an integrator 12 and signals, exceeding the threshold value, among AE signals which are converted into voltages are integrated in the integrator 12. Respective integrated values are accumulated by a counter 8 and it is corresponding to the sum of parts in which diagonal lines are described. Accordingly, the progressing state of abrasion or the degree of damage, such as defects or breakage, may be detected even while working by monitoring the accumulated value.