the predetermined value is set to be 50 to 70 of a critical strain even when a load given to the cutting tool is increased by adjusting a voltage impressed upon piezoelectric element p deformation of the tool rest 531 is restricted by the restricting member 532 so that the output from the force sensor 504 is not increased more than a predetermined value when the cutting condition is normal and cutting force of cutting tool a is in a predetermined range elastic deformation of the force sensor 504 is restricted in the range of gap g therefore cutting is continuously conducted while the force sensor 504 is not brought into contact with each restricting member 532 in the case of an abnormal cutting condition or an accident the cutting force given to the cutting tool exceeds a predetermined range so that a strong force is given to the force sensor 504 in the aforementioned case the force sensor 504 is deformed and gap g becomes zero so that the force sensor 504 is brought into contact with the restricting member 532 as a result the deformation of the force sensor 504 is restricted by the restricting member 532 accordingly the force sensor 504 is not deformed exceeding a predetermined value so that the damage of the force sensor can be prevented the aforementioned example shows a mechanism to prevent the damage of the force sensor 504 in the direction of a principal force with regard to the directions of feed force and radial force the damage of the force sensor 504 can be easily prevented in the same manner in the case of a force sensor in which.