To provide a cutter defect detecting method capable of surely detecting a defect of a cutter in a short time, while rotating the cutter without stopping it when detecting the defect of the cutter of an NC machine tool.

This cutter defect detecting method for the numerical control machine tool is characterized in that a distance detecting means measures a distance d to the cutter 3, while rotating the cutter 3 having multiple blade parts 3a, compares the measured data with similar measured data obtained from the blade 3 having no defect, and detects presence/absence of the defect of the blade 3 by difference from the both measured data.