A nut to be fastened onto a bolt, the nut comprises: a nut main body having a substantially cylindrical shape and constituted by a plurality of nut segments obtained through segmentation of the nut main body in a circumferential direction; an outer cover configured to cover an outer circumference and one end face of the nut main body; an inner cover comprising a restraining plate at one end portion of the inner cover, the inner cover being accommodated in the outer cover and configured to cover the outer circumference of the nut main body; and a ring-shaped spring disposed on an inner wall of the nut main body and configured to bias the nut main body to cause the nut main body to expand radially toward an inner wall of the inner cover, wherein a protrusion protruding in a circumferential direction is formed on the inner surface of the inner cover, a groove is formed in a circumferential direction on an outer surface of the nut main body in a manner in which the groove is capable of engaging with the protrusion and accommodating the protrusion, and the protrusion has a slant surface and the groove has a slant surface, the slant surfaces being configured to contact each other on the same side in the axial direction, characterized by further comprising a restraining ring configured to abut against the other end face of the nut main body for restricting movement of the nut main body in an axial direction in the outer cover while allowing the nut main body to only expand and contract in a radial direction in the outer cover, and the restraining plate being configured to restrict a position of the restraining ring. the nut comprises: a nut main body having a substantially cylindrical shape and constituted by a plurality of nut segments obtained through segmentation of the nut main body in a circumferential direction; an outer cover configured to cover an outer circumference and one end face of the nut main body; an inner cover comprising a restraining plate at one end portion of the inner cover, the inner cover being accommodated in the outer cover and configured to cover the outer circumference of the nut main body; and a ring-shaped spring disposed on an inner wall of the nut main body and configured to bias the nut main body to cause the nut main body to expand radially toward an inner wall of the inner cover, wherein an inner cover comprising a restraining plate at one end portion of the inner cover, the inner cover being accommodated in the outer cover and configured to cover the outer circumference of the nut main body; and a ring-shaped spring disposed on an inner wall of the nut main body and configured to bias the nut main body to cause the nut main body to expand radially toward an inner wall of the inner cover, wherein a protrusion protruding in a circumferential direction is formed on the inner surface of the inner cover, a groove is formed in a circumferential direction on an outer surface of the nut main body in a manner in which the groove is capable of engaging with the protrusion and accommodating the protrusion, and the protrusion has a slant surface and the groove has a slant surface, the slant surfaces being configured to contact each other on the same side in the axial direction, characterized by further comprising a restraining ring configured to abut against the other end face of the nut main body for restricting movement of the nut main body in an axial direction in the outer cover while allowing the nut main body to only expand and contract in a radial direction in the outer cover, and the restraining plate being configured to restrict a position of the restraining ring.