To provide a low-cost nut having a locking function and preventing the damage of a lens.

A locking nut comprises a given female screw 2 formed; and a screwless hole 4 consisting of a screw-in part 4a having diameter equal to or lower than the effective diameter of a female screw 2 and a vertical groove part 4b more escaping to the outside than the screw-in part 4a formed on the central line of the female screw and is formed of a resin member 3. Thereby, since, during fastening, the locking nut is formed of a resin member, an elastic force is provided and an axial fastening strength is sufficiently ensured. Besides, since the screwless hole of the resin member has diameter lower than the outside diameter of a screw, during fastening, a thread is formed by the screw itself, and the outer surface of the screw is fastened by the elastic force of the resin member to prevent unfastening of the screw. Further, since a need for a T-type washer is eliminated by an integrally molded bushing part 5, assembly workability is improved, an assembly cost is reduced, and the damage of a lens is also prevented from occurring.