To easily achieve the visual checking performance of a fastening state by a bolt.

A first projection 17 of a disc spring washer 16 and a second projection 19 of a flat washer 18 are combined in an engaged state, a shaft part of a bolt 13 is inserted therethrough, and the bolt 13 is rotated to fasten fastened members 11, 12 by the bolt 13 and a nut 15. When the rotation of the bolt 13 is advanced, the disc spring washer 16 is compressed and deformed to be flat, the engagement of the first projection 17 and the second projection 19 is released by the deformation, and when the bolt 13 is further rotated and fastened, the phase difference in the rotating angular direction is generated between the first projection 17 and the second projection 19. The fastening state can be confirmed by visually checking the releasing of the engagement of the first projection 17 and the second projection 19, and the phase difference.