To provide a crossbolt combination nut having an effective locking function.

A pair of nuts are used screwed with a cross external thread with a left external thread and a right external thread cut on the outside surface of the same shank. In the pair of nuts, a left internal thread is cut on the one nut, and a right internal thread is cut on the other nut. The upper part of the right internal thread is provided with an annular protrusion having an inclined outer peripheral surface reduced in diameter toward the upper part, and the lower part of the left internal thread is provided with an annular recess having an inclined inner peripheral surface reduced in diameter toward the upper part. An angle difference is provided between the inclined outer peripheral surface and the inclined inner peripheral surface, and when both the nuts vertically abut against each other, an annular protrusion upper edge and the inclined inner peripheral surface, or an annular recess lower edge and the inclined outer peripheral surface contact with each other. One nut is turned relative to the other nut to bring both the nuts into pressure contact and to integrally hold both the nuts. The pressure screwed state of the nuts to the cross external screw can thereby be maintained.