To provide a locking nut, engaged with a bolt to rotate relatively to the bolt, not requiring a special tool to simplify the operation, having no fear of damaging a bolt male screw part to obstruct a practical use, displaceably detent-locked in a designated position on the bolt with high reliability, and used in various fields.

This locking nut R1 has a built-in coil spring 1 providing a nut female screw part, and includes a spring operating part (a first ring part) 2 movable in the expansion and contraction direction of the coil spring 1 by the operation from the outside, which compresses the coil spring 1 by the movement in the above direction to change the spiral pitch of the coil spring 1 into a spiral pitch P' deviated from the spiral pitch P providing the nut female screw part.