Abstract: Conventional bolt-rotation preventing devices are designed to be handled from both bolt and nut sides. Therefore, in locations where the device can be handled from only one side thereof, it has been very difficult to screw a bolt with a nut as well as to prevent the bolt from getting loose. A screw-rotation preventing device includes; a main portion (7B) having a through-hole (7A) for passing the shank of a screw (4) having a shank with a male thread on the external surface thereof and a head (4A) with a diameter larger than that of the shank, the through-hole (7A) having a diameter smaller than that of the screw head (4A); an engaging portion (7C) for engaging with a side of an attachment member (3) or a mounting member (2) or a side each of both an attachment member (3) and a mounting member (2); and a head-pressing member (7D) fixed to the main portion (7B) so as to sandwich the screw head (4A) between itself and the main portion (7B) to clamp down the head (4A).