To prevent erroneous defect detection even when cutting torque is fluctuated by the eccentricity of a tool by providing a defect detecting means making the final defect detection based on the output signal of a defect detecting means judging a defect based on different judgment indexes.

Torque generated by an end mill is detected by a torque sensor in a cutting device, and a first defect detection section 60 obtains the maximum value of the cutting torque generated on the edge of a rotary cutting tool based on the output signal of an A/D converter 6 and judges the defect of the rotary cutting tool via this value. A second defect detection section 70 wavelet-converts the output signal of the A/D converter 6 and judges the defect of the rotary cutting tool. A third defect detection section 80 makes the final defect judgment of the rotary cutting tool based on the judged result of the first detect detection section 60 and the signal of the second defect detection section 70.