Abstract: A self-lock screw which has a deep slit so as to facilitate the elastic deformation of the flanks of a thread and enhance the locking effect and which can be produced without the need for skill of a high level and dexterity. A slit (3) which separates the flanks (21) on both sides of a thread (2) along the crest of the thread is provided, and a broad bottomed concave portion (22, 23) is formed from the vicinity of the root of the thread (2) in a shape given by inwardly scraping the extended surfaces of the flanks. Since the slit at the crest of the thread becomes deep by providing the broad bottomed concave portion in the vicinity of the root of the thread, the flanks are ready to be elastically deformed, so that the spring back effect is enhanced and the effect on preventing the screw from loosening is therefore increased.