To provide a nut fastening method capable of achieving the complete detent for loosening by surely bending and connecting an annular part projected from an upper face of a nut, into a groove part formed on a fastened part, after completing the fastening.

The thin annular part 41 having a diameter approximately same as a screw part 1a of a bolt body 1 is projected upward from the upper face of the nut 4, a plus-shaped groove part 1b extended in the radial direction is formed on a side end part of the screw part 1a of the bolt body 1, a bending connection jig 6 is positioned by utilizing the groove part 1b after the completion of fastening, and the annular part 41 of the upper face of the nut 4 is bent and connected in the groove part 1b with the directionality based on the jig 6.