Abstract: A lock nut with a nut and an annular flat plate made of resilient material, the annular flat plate having a circumferential periphery and an arcuate protrusion, wherein the circumferential periphery is secured on one surface of the nut, and the arcuate protrusion is extended inwardly from an inner circumference of the circumferential periphery integrally over one-half or more of the inner circumference so as to contact the threaded surface of a male thread onto which the nut is screwed.

Abstract: A cover of a nut according to the present invention is constituted of a resilient body, having a cavity shaped so as to be prevented from turning by an operating means formed on the nut for convenience of turning the nut adapted to be engaged with the bolt and being equipped with a tubiform basal portion receiving the periphery of the nut engaged with the bolt to be fitted within said cavity and a hollow swelled portion extending integrally from one of the ends of said basal portion and closed at an end located far from the basal portion. Said resilient body is provided with a disengagement-proof means preventing the cover applied over the nut from slipping out of the nut after displacing in the direction of the axis of the nut.