To provide a nut simply mountable to a bolt only by pressing it and allowing retightening.

One end side and the other end side in the axial direction of an inner hollow part 2 of a nut main body 3 provided with a polygonal circumferential wall are formed into a large diameter part 2a and a small diameter part 2b, respectively, and a tapered hole part 2c formed by gradually reducing its diameter from the large diameter part 2a toward the small diameter part 2b are formed between them. Guide pieces 4 are projected on the tapered hole part by arranging them in its circumferential direction. A screwing piece 5 with a female screw part (a) formed on its inside surface is movably engaged with a part between the guide pieces 4, 4. A pressing spring 8 for always energizing the screwing piece 5 in the small diameter direction of the taper hole part is housed in the main body 3.