To provide a tool used for fastening and loosening a bolt and a nut.

In this co-rotation preventing tool of a bolt and a nut, as shown in Fig. 5, an alligator 1b of a base plate having an opening width under hexagonal cross of the bolt or the nut is provided one base plate part 1a of an L-shaped holder 1, a receiving plate 2 is provided on the outer side surface 1e of the base plate, the other holder side wall part 1d is provided with a magnet 3 mounted on the inside surface 1f of the side wall, a top plate alligator 4b having an opening width above a hexagonal cross of the bolt or the nut at one top plate part 4a is provided on the outside surface 1g of the side wall, and an L-shaped support plate 4 provided with an elongated hole 4d in the other side wall part 4c is fitted by a wing bolt 5. This tool is used in fastening and loosening the bolt or the nut.