A locking screw including a male screw and a female screw, the male screw being screwed into the female screw, the male screw including male threads and the female screw comprising female threads, wherein a male thread comprises a crest side flank surface and a root side flank surface connecting the crest side flank surface and a root of the male thread, a slope of the crest side flank surface is smaller than a slope of a female flank surface of the female thread, a slope of the root side flank surface is larger than the slope of the female flank surface, a connected portion of the crest side flank surface and the root side flank surface interferes with the female thread, and a clearance is formed between the root side flank surface and the female screw when the connected portion interferes with the female thread.