PURPOSE: To provide a fastening method by a covered nut that can prevent a nut from being self-locked or left untightened or can prevent the scattering of a jacket member mounted on the nut.

CONSTITUTION: In a covered nut A, a jacket member 2 mounted on the periphery of a nut 1 is contracted to adhere closely, and one side part of the jacket member 2 is formed into a diameter smaller than the internal thread minor diameter of the nut 1 to form a locking part 6. The covered nut A and an external thread body 4 are screwed with each other from the other side of the locking part 6, and the locking part 6 is screwed with the external thread 5 of an external thread body 4 protruded from one side part of the nut 1, thus integrating the locking part 6 with the external thread 5.