To improve a locking effect of a bolt/nut for mischief by providing a cover pipe covering a peripheral surface of a tightening nut screwed onto a tightening bolt such as a steel tower and a locking nut preventing the fall-off of the cover pipe and holding the cover pipe in a state that the peripheral surface of the tightening nut is covered because the locking nut is screwed onto the tightening bolt.

When flanges 22, 22 for connection of a stud 21 are connected and fixed by using a tightening bolt 23, a cover pipe 1 is fitted into outer peripheries of a tightening nut 25 and a lock nut 24 and a locking nut 4 with flange is screwed into a tip screw part of the tightening bolt 23 projecting upward of the lock nut 24. Next, the locking nut 4 is tightened by engaging a projecting piece 12 of a special tool 11 to an engagement groove 7 formed on a flange 5 of the locking nut 4, inserting a bar-shaped body of a shaft part of a driver into an inserting/passing hole 13 of a tool main body 11a and rotating the body, the locking nut 4 is tightened. As a result, the outer peripheral surfaces of the tightening nut 25, the lock nut 24 and the locking nut 4 are covered with the cover pipe 1, and fall-off and locking of the cover pipe 1 are performed.