PURPOSE: To improve slack preventing effect in a locking nut structure screwing to a bolt inserted through a workpiece for fastening the workpiece.

CONSTITUTION: This structure is constructed of a main nut 1 and a sub-nut 2. In the main nut 1, a projection part 12 provided with an external thread part 120 and a screw hole 13 are arranged, and screw thickness in the effective diameter of the screw hole 13 is set to be larger than a difference between a radius of the thread root of the screw part 13 and that of the external thread part 120, while a radius of thread root of the screw hole 13 is set to be smaller than that of the external thread part 120. In the sub-nut 2, a large-diameter first screw hole 210 and a small-diameter second screw hole 220 are arranged continuously. The external thread part 120 and the first screw hole 210 are formed in the same size, while the screw hole 13 and the second screw hole 220 are formed in the same size. A pitch of the external thread part 120 is set to be smaller than that of the screw hole 13.