To avoid reduction of service life by moderating stress concentration at an opening edge of a screw groove when loaded with heavy load, to prevent foreign matter inclusion into a rolling passage such as a burr at the opening edge of the screw groove, and to improve safety when a worker handles the screw.

A curved part 7b of a projecting curved surface smoothly continuing from the inner surface 7a of the screw groove is formed at the opening edge of the screw groove 7 of a screw shaft 2. A curved part is formed also for a screw groove 8 of a nut 3 similarly. The curved part 7b is continued to the outer diameter surface 2a of the screw shaft 2. When a chamfer is disposed at the opening edge of the screw groove 7, the curved part 7b may be disposed between the chamfer and the inner surface 7a of the screw groove. The curved part 7b may be ground by a grinding wheel together with the screw groove 7, and may be machined by grinding the corners with a sandpaper or the like after molding the screw groove 7.