To facilitate work of checking a nut for fastening to avoid failure of fastening the nut, prevent a nut from loosening and facilitate inspection of fastening after a large number of washers and nuts are fastened, in various fastening work of fastening materials employing bolts, nuts and washers in various industries.

A fixing material 7 is fixed by an penetration groove 10 on the nut side and an extrusion port 4 in a washer with the passage of time by including an indentation in a washer, penetration grooves for a binder and extrusion port in a washer and nut. By so doing, the washer and nut are made to be a loosening preventer. Likewise, the fixing material 7 extruded from an extrusion groove 5 in the washer is extruded onto a fastening material and converted to a binder to be a loosening preventer. The exposure of the binder allows the visual confirmation of the completion of fastening.