To provide a lock nut which has a simple structurere to reduce manufacturing cost, is easily operated and further can improve a fastening effect when being fastened to a bolt.

A lock nut is a double nut constituted of a first nut in which a screw hole is penetrated on the center in the axial direction and a polygonal part is formed on the outer circumference and a second nut provided on the first nut. In the double nut, the first nut includes a tapered part which is a screw hole outer circumference and is formed on a portion besides the polygonal part and cutout parts formed on the tapered part. Therein, the cutout parts are formed at at least one portion on the tapered part. Further, the second nut is configured such that the screw hole is penetrated on the center in the axial direction and a second tapered part having the tapered part of the first nut arranged on a part of the screw hole is formed.