Abstract: A lock nut capable of improving a fastening force of a fastening nut and the lock nut is provided. The lock nut includes a body having a threaded throughhole and a groove formed at a circumferential edge thereof. A plurality of projections are formed in the groove and each includes an outer side face directed toward a center of the body from a border between a seat surface and the groove. An inner side face serves as an extension of an inner face of the threaded throughhole. A depth of the groove is such that if the projections are crushed when the lock nut is fastened against a bolt, they do not enter a space between a seat surface on the side of the body of the fastening nut and the seat surface of the lock nut body.