To protect a nut even from external force while preventing looseness of a fastened nut by bringing both surfaces of the nut and the cap part into pressure contact with a bolt, and forcibly screwing the nut part up to shearing from the cap part.

Both surfaces of a nut 3 and the cap part 21 are brought into pressure contact with a bolt in the state of (A), and when the nut part 23 is push rotated so as to be forcibly screwed by gripping the nut part by a wrench, the slitting 22 part shears, only the cap part 21 functions to prevent looseness of the nut 3 by remaining on a bolt 2, and prevents falling-off of a sleeve 10, and the sleeve 10 functions to protect the nut 3 from energization of external force though rotatable around the nut 3 in a (B). In such constitution, exertion of external force is prevented by the presence of the taper surface of the cap part to prevent looseness of the nut 3, and the nut 3 is protected from physical contact and impact, that is, energization of external force capable of rotating/ relieving the sleeve 10 and preventing the exertion by the presence of the sleeve 10 put on the periphery of the fastened nut 3.