To perform simple and sure locking by screwing a fastening ring capable of being expanded/contracted to a bolt, screwing a nut to the bolt, thereafter inserting the fastening ring into the nut in a press contact manner, and generating frictional force between the bolt and the fastening ring and between the nut and the fastening ring.

A fastening ring 2 capable of being a little expanded/contracted because of having a slit 23 is mounted on a bolt 3, screwed in until it is contacted with a member T to be fastened, and thereafter a nut 1 is screwed in. Even when the edge 13 of the spread hole 12 of the nut 1 is brought in contact with the fastening ring 2, because the fastening ring 2 is rounded at the edge, the fastening ring 2 is smoothly put in the spread hole 12 and inserted gradually into the spread hole 12 in a press contact manner by further rotationally moving the nut 1. By contacting the nut 1 with the member T to be fastened, the screwing-in is completed, and strong frictional force is applied between the nut 1 and the fastening ring 2 and between the fastening ring 2 and the bolt 3. Consequently, even if repeated stress or large impact force is applied to them, looseness is not generated.