To provide an excellent nut caulking tool capable of keeping the finishing quality of caulking portion at a predetermined level irrespective of the skill of a worker, considerably suppressing any human errors, and reducing a working space.

The nut caulking tool fastens a lock nut 1 to a shaft 2 (a male screw parts) having a groove part 5 at an adequate position in the circumferential direction, and caulks a part of a caulking ring 4 at a top of the lock nut 1 so as to be bitten in the groove part 5 of the shaft 2. The nut caulking tool has a socket type body part 11 which is covered over the lock nut 1 from the axial direction of the lock nut 1 to be housed in and externally attached thereto. A claw part 12 is provided in the body part 11, which caulks the body part so that a part of a caulking ring 4 to be bitten in the groove part 5 of the shaft 2 when the body part 11 is covered over the lock nut 1 and pushed in to the butting position.