PURPOSE: To ensure rupture of a fixed torque and drop of a display plate by providing a pair of nut parts in series through a fixed torque rupture part, by separating the display plate fitted to the rupture part at one point, and by fitting a pair of engagement pieces protruded from each separation end part in the reverse direction, in the groove of each nut part.

CONSTITUTION: A rupture nut device is screwed into a bolt 40, and a nut part 12 is seated on the surface of a material 30 to be fastened. When a nut part 11 is rotated by a tool, a rupture 13a is generated an a fixed torque rupture part 13 between the nut parts 11 and 12, and when the nut part 11 is further rotated, relative rotation displacement is generated between the nut parts 11 and 12, and fixed torque fastening and double-nut formation are achieved. In the movement from the rupture 13a to the relative rotation displacement between the nut parts 11 and 12, an engagement piece 21 fitted into the groove 15 of the nut part 11 and an engagement piece 22 fitted into the groove 16 of the nut part 12 are pulled in the reverse directions, and are expanded from the separation part of a surface plate 20, while the rupture occurs at a desired part in the circumferential direction, and the display prate 20 is dropped from the nut main body 10. Complete fastening can thus be judged.