To quickly mount a screwing nut on the projection end of the bolt penetrating the fastened body.

A screw hole 53 is provided on the front inner surface of a pair of levers 50, 50 oppositely arranged with a spherical body 40, and a cut hole 58 is also provided on the rear part of the screw hole. A notch 55 is provided on the rear inner surface of the pair of levers 50, 50. The pair of levers 50, 50 on which a spring 60 urged in the opening direction to the two split screw holes 53 is arranged, are arranged in the outer cylinder 70. A pin 80 penetrating the outer cylinder 70 in the perpendicular direction to the central axis is displaced along the direction of generating line of the long hole 73. In the fastening nut with one step operation, the screw hole 53 is opened to come to a state where the bolt 12 is inserted at the retracted position of the outer cylinder 70 and the screw hole 53 becomes to a state where the bolt 12 is screwed at the advanced position of the outer cylinder 70.