To accurately detect a fracture of a cutting tool even in case where a change of the cutting torque value with the fracture of a cutting tool is small by providing a fracture detecting means for judging the generation of fracture on the basis of a power spectrum of the frequency synchronized with the number of revolution of a spindle.

A peal value detecting part 8 detects a peal value Pp of a powder spectrum, the thereafter, outputs the data of the peak value Pp to a comparing part 10. The comparing part 10 compares the peak value Pp with a fracture judging power spectrum threshold value Pk, and judges whether the peal value Pp exceeds the fracture judging power spectrum threshold value Pk or not. In case where the peak value Pp exceeds the fracture judging power spectrum threshold value Pk, the comparing part 10 judges YES, and outputs the signal S1 to a signal generating part 11. The signal generating part 11 outputs the fracture detecting signal Sk for showing that the fracture of a cutting tool is detected to a control part 3.