Abstract: Provided is a fall-off preventing element and a loosening preventing nut, which, by a wire of a coil spring, with a free lower end, being fitted onto a thread groove of a bolt, can prevent fall-off of a nut or loosening of a nut. A fall-off preventing element is screwed onto a bolt and includes a coil spring portion and a cylindrical main body affixing an upper end of the coil spring portion and having an opening formed therein in which a lower end of the coil spring portion is latched. The coil spring portion is made smaller in its inner diameter than the outer diameter of a leg portion of the bolt. The lower end of the coil spring portion is latched in a diameter-expanded state to the cylindrical main body, and the lower end of the coil spring portion is enabled to move in a diameter-expanding direction.