PURPOSE: To prevent a bolt from being loosened by providing a tapered hole and a slit for the axial through hole of a bolt main body, forming a notch and a taper at the head section of a deforming bolt in the through hole, and thereby threadedly engaging a nut with the shaft end section of the deforming bolt within the recessed section of the head section of the bolt main body.

CONSTITUTION: When a nut 3 is rotated after a bolt main body 1 has been installed in a bolt hole 14, the tapered head section 9 of a deforming bolt 2 disposed within the tapered hole 7 of the bolt main body 1 is moved to the head section 4 of the bolt main body 1. This constitution thereby allows the inner wall surface of the tapered hole 7 to be expanded outward by the head section 9. In this case, the head section 9 allows an overhang section 13 due to a notch 12 to be engaged with the slit of the bolt main body 1, so that a deforming bolt 2 is prevented from being rotated together with the nut 3. The outward expansion of the shaft end section of the bolt main body 1 permits the screw thread of the shaft end section to bite in the screw thread of the bolt hole 14, and prevents the bolt main body 1 from being rotated thereafter, so that the bolt main body is thereby prevented from being loosened.