Comprised of an antenna base in which a protruding portion is formed on a bottom face and tapered faces are formed on shoulder portions of the protruding portion, a legged washer in which leg portions whose tip ends have an external diameter larger than the internal diameter of a mounting hole and which has an elasticity and leg portions, which are bent into a crank-like section so that the diameter of the tip ends is increased and in which the inside face of the crank portion makes contact with the tapered faces when the axial length of the bolt is decreased with respect to the antenna base by tightening the bolt so that the tip ends are expanded, are formed integrally with the ring portion, and a bolt which is fastened to the protruding portion of the antenna base through the legged washer.