To make a ring easily visible and to easily find the abnormality of a bolt by applying a red color or a yellow color which is a conspicuous color to the ring integral with a tube section covered on the apex of the bolt.

A ring 3 integral with a tube section 2 made of a soft synthetic resin is coupled with the apex of a bolt 1, and the ring 3 is colored in conspicuous red or yellow. The inside of the tube section 2 is coated with a strong adhesive, and the tube section 2 is covered on the apex of the bolt 1 so that the whole ring 3 is visible from the advancing direction of a train. The whole ring 3 is directed in the advancing direction of a worker, and a visual inspection can be fully made. When an abnormality occurs on the bolt nut, it can be easily found.