Abstract: A device for preventing loosening of a prismatic member, such as a fixing nut, screw head or bolt, and particularly a lock nut of a wheel of an automobile vehicle, includes a cap adapted to cover the head of the prismatic member. The cap includes a transverse bottom and a cylindrical skirt in one piece with the bottom and a mechanism means for locking the prismatic member received in the cap. The locking mechanism for the prismatic member in the cap are constituted, in combination, by a ramp inclined with respect to the longitudinal axis of the cap, placed inside the skirt, a shim placed between the ramp and the lateral face of the prismatic member sliding both on the ramp and on the lateral face of the prismatic member, and an arrangement for longitudinally displacing the shim in contact with the ramp and the lateral face in order to exert a transverse effort for locking the prismatic member.