To use a bolt or a nut and a washer in a place where inconvenience arises such as loosening after a main body and the like are fastened by an assembly of the bolt or the nut and the washer, a place where is not necessitated for loosening again, or a place where inconvenience arises when they are loosened by mischief.

Several numbers of small projections are formed on a lower surface of a washer, a surface where is brought into contact with a bolt or a nut of an upper surface is formed in a wave shape like stairs, and a lower surface of the bolt (the nut) is also formed in the wave shape like the stairs. When the bolt and the washer are assembled and used, and the bolt is fastened, the projection of the washer collides with a main body. Furthermore, when the bolt is fastened, the projection touches the main body. Finally, the washer is caught, springing by a step difference so a to stop rotation. The washer and the bolt are not rotated in a loosening direction because of a stair shaped structure. As a result, when the washer and the bolt are fastened once, they are not loosened again.