To perform screw-locking without requiring any operation other than thread fastening, in a clamping tool using screw threads, like bolt and nut.

With a bolt 1 having a male screw 4, a crossed split groove 5 is formed on the tip side from a female screw threading part 4a at which a female screw 6 is positioned when an object to be clamped is clamped. On the other hand, a nut 2 having the female screw 6 is formed into a shape of box nut which surrounds the tip part of the bolt 1 when an object to be clamped is clamped, and forms a projection 8 on the inner bottom part. Thus, the projection 8 formed in the nut 2 cuts into the tip of the bolt 1 simply by performing thread fastening. The tip part of the bolt 1 is split and spread, and thus the screw threads of the bolt and the nut are brought into pressurecontact with each other to perform locking.