To provide a lockable nut capable of constantly obtaining a locking function.

This nut has a lock part 2 provided with a contraction part 3 extensionally mounted in the axial direction of a nut body 1 to extend a tapped hole, a pressing member 4 formed separately from the contraction part 3 and fitted to a tip-side outer periphery of the contraction part 3, and a drop- prevention structure 5, and the pressing member 4 is slid to the basic end side of the contraction part 3 to press the contraction part 3 to contract a diameter of the tapped hole of the contraction part 3. The drop-prevention structure 5 locks the pressing member 4 rotatably and axially slidably to the contraction part 3, and prevents the drop of the pressing member 4 from the contraction part 3.