To provide a double nut type looseness preventive nut capable of easily working a small cross-sectional rupture part having the uniform cross-sectional area, making shear fracture force constant and capable of obtaining prescribed fastening torque by changing a shape of expanding slots arranged in a nut body.

A pair of mutually opposing expanding slots 3, 4 are arranged toward the axis from an outer peripheral surface in an intermediate part of the nut body 2, and the rupture part 5 extending in the radial direction is formed between both expanding slots 3, 4. The pair of expanding slots 3, 4 are arranged so as to be slightly dislocated in the shaft direction of the nut body 2, and are shear-fractured by the prescribed fastening torque, and since an upper side nut body 2A among separated two nut bodies 2A, 2B inclines to the axis by further fastening force, the strong looseness preventive action can be exhibited by the looseness preventive action of mere double nuts.