A fastening socket B includes an inner socket portion including engaging means for engaging a nut or the like, and an outer socket portion for engaging a washer A with an outer edge portion 2, which includes a tapered surface and engaging teeth. The tapered surface is gradually inclined outwardly and toward a bottom surface of the outer edge portion. The inner socket portion 21 and the outer socket portion 22 are coupled together with a mechanism so as to be rotated in opposite directions. The outer socket portion is provided with engaging claws for engaging the engaging teeth, and a tapered bottom end surface of each of the claws 24 is gradually inclined radially inwardly and toward a base end side. A surface of a bolt head that is brought into contact with a surface of objects is provided with frictional coefficient increasing treatment.