To provide a nut capable of being used even in a narrow fitting width, and excellent In the locking effect.

A groove part 1 notched from a bearing surface 5 side in the axial direction is formed on a part of a threaded hole of a main nut 2, and a locking member 3 in which ridges continuous to ridges 4a of the main nut 2 are received by the groove part 1 slidably in the axial direction. A projection part to be projected on the bearing surface 5 side is provided on an end part of the bearing surface 5 side of the locking member 3 in the pre-tightening condition to constitute a nut. When a bolt 11 is screwed in the main nut 2 and tightened in a condition where the locking member 3 is received, the projection part is abutted on a tightening member, and the locking member 3 slides by the groove part 1 in the axial direction of the threaded hole.