To provide a nut which is so hard that the removal of a nut by an outsider is nearly impossible.

This nut is composed of a nut body 1 whose outer periphery is circular and a split ring 2 made by a sheet spring and a ring shape groove 4 is formed on the center of a width direction on the outer periphery surface of the nut body 1 and big diameter parts 5, 6 whose diameter is bigger than the diameter of the groove 4 are formed on both sides of the groove 4 and an engaging hole 7 which can engage the engaged projection of the wrench for nut rotation is formed at the equal interval in the peripheral direction on the bottom of the groove 4 and the split ring 2 has an inner diameter inserted turnably in the groove 4 and its outer diameters 5, 6 are bigger than big diameter parts 5, 6. Thereby, even if it is grasped by a pipe wrench, the split ring 2 is only rotated and the nut body 1 is not rotated. Therefor, the outsider can not remove.