To provide a lock nut with a simple structure, reducing manufacturing costs, facilitating operation, improving a fastening effect when fastening a bolt, and capable of keeping two members connected.

A lock nut 1 includes a first nut 10 pierced in an axial center by a screw hole 11 and formed in a circumference with a polygon part 12, and a second nut 20 provided on the first nut. The first nut includes a cylindrical part 13 formed in a portion in a screw hole circumference and excluding the polygon part, a protruding part 14 formed on a tip of the cylindrical part, and a slit part 15 formed on the cylindrical part. The slit part is formed in at least one portion of a taper part. The second nut is pierced in an axial center by a screw hole, and is formed in a circumference with a polygon part.