To simply and securely control a tightening torque and lock the loosening.

In a fastening nut 1, a screw that is tapered with the diameter increased in an axial direction 6, is provided between an outer member 2 and inner member 4, and they are connected with a torque control pin 5. In the course of tightening the outer member 2, the torque control pin 5 brakes when it reaches a prescribed torque to make a separation between the outer member 2 and inner member 4. The outer member 2 locks on the inner member 4 because of the taper, and the diameter of the inner member 4 decreases to tighten such as a screw part of a bolt by a female screw part 3 and lock the tightening. The brake of the torque control pin is known by shock or can be confirmed by the displacement of a paint 10 for torque confirmation.