To provide a nut enhancing an effect to prevent loosening, falling-out of a male screw and play between the male screw and the nut.

An engaging member 3 comprising a plate material with elasticity and containing a plurality of engaging pieces 6 is fitted to a nut body 2. In an initial state, the engaging pieces 6 are protruded outward from a flange 2a side of one side of the nut body 2 with regard to the axial direction of 3 nut 1. When the nut 1 is fastened, the engaging pieces 6 are axially pressed by a fastened matter 10 or the like, so that the engaging pieces 6 are deformed and then their points 6a are moved to a side of an axial line (an inner side) of a threaded hole 4 to get strongly into thread grooves 11 of the male screw 8.