To provide a screw and nut loosness stopping mechanism which can stop looseness surely and can conduct screw return without destruction.

Inclination surfaces 34 and engagement surfaces 35 are respectively provided on the outer periphery surfaces of a screw 30 and a nut, and at a plate 14, elastic engagement claws 54 are provided directly or through clips 50. As a result, at the time of the screw 30 and the nut being screwed in, there is no fear of trouble being brought about for the screwing-in or the like of the screw 30 and nut, as the elastic engagement claws 54 slide bendingly on the inclination surfaces 34. The screwing-in of the screw 30 and the nut is completed, and the screw 30 and the nut begin to be returned, then the elastic engagement claws 54 are elastically engaged with the engagement surfaces 35, and the loosening of the screw 30 and the nut can be prevented. Moreover, by releasing the elastic engagement of the elastic engagement claws 54 with the engagement surfaces 35, screw return can be accomplished without destroying the screw 30 and the nut.