To provide a lock bolt and nut for preventing looseness between a fastening hexagonal nut and a lock hexagonal nut by fastening the hexagonal nuts together with mutually reverse turn.

A step is provided on the diameter of one round steel, a right-turn thread is screw-cut in one round steel having a large diameter, and the fastening hexagonal nut is screwed thereinto. An adjusting cylinder is passed through the step portion of the bolt, a left-turn thread is screw-cut in the other round steel having a small diameter, and the lock hexagonal nut is screwed and fastened thereinto so that both hexagonal nuts are bitten into the rough contact face of the adjusting cylinder.