To provide a device capable of suppressing theft without providing any firm locking device by solving problems that a tightening device for fitting an article by a bolt and a nut can be easily attached/detached by a regular tool such as a spanner, a wrench and a bias clip because of the hexagonal contour or the knurling on the outer circumference, and suppressive effect against theft is less.

The nut has a small circular contour part on the inner side and a large circular contour part on the outer side, a ring member comprises a small diameter locking part which is freely fitted in the small circular contour part so as to cover the outer circumferential surface of the nut, and a large diameter ring part which is freely fitted in the large circular contour part, and the nut is tightened to the bolt so that the small circular contour part is on the tightening side with the ring member combined with the nut.