To prevent the removal of nuts by a mischief after once tightened, in the nut used at the construction time of a steel toward or a bridge.

A nut having a female screw 2 on the axial center divides the axial center to two vertically and formes the female screw 2 on its lower part and the loose hole 3 on its upper part and the position corresponding to the female screw 2 is formed in a flat conic shape and set to a tightening part 5 and also the position corresponding to the loose hole 3 is set to a hexagonal torque input part 4 and a notch part 6 and twisted off by a new rotation after tightening the torque input part 4 and a notch part 6d for separating the tightening part 5 from the torque input part 4 is formed between these tightening part 5 and torque input part 4. After tightening, only the tightened part 5 is left and maintains the tightened state, but as the tightening part 5 is a flat and conic shape, the jig for rotation of a wrench can not be engagingly locked thereby not allowing removal.