To prevent the drawing-out of a nut from a bolt by forming cylindrical part in a nut protection cap of magnetic material, constructing it of a large diameter bore cylindrical part and a small diameter bore cylindrical part, sticking a permanent magnet in a stepped part between them, and attaching the nut protection cap to a already installed fastened member by means of magnetic force.

When, in a pipe steel tower, pipes P1, P2 are connected together via opposed flanges F1, F2 by means of a bolt, a protection cap 1 is put over an already arranged nut, and the protection cap 1 is sucked to the flange F1 by means of the end edge of a large diameter cylindrical part 1a. In the protection cap 1, its outline is constructed of the large diameter cylindrical part 1a and a small diameter cylindrical part 1b, and a stepped part is formed of the large diameter bore and a small diameter bore in the corresponding cylindrical parts respectively. In the stepped part, a permanent magnet 2 is stuck, and a line of magnetic force is formed between the flange F1 face and it. In the end face of the small diameter cylindrical part 1b, a through hole 1c is bored with a slight clearance between a flange bolt B and it, and around the through hole 1c, a small hole 1d for an uncapping tool is bored.