To lock a male screw part in a female screw part to be prevented from being disengaged when a screw is inserted into a nut or the like by forming the outer diameter of a screw shaft part to be smaller than the inner diameter of the female screw part and the angle of a screw thread in the top of the male screw part to be larger than the angle of a screw thread in the base part.

A screw is composed of a screw head and a shaft part 12, and the outer diameter d1 of the shaft part 12 before form rolling is smaller than the inner diameter d2 of a nut 20. This shaft part 12 has a male screw part 13 formed only in its tip side by form rolling except its neck lower part 12a, and regarding a screw thread 14 constituting the male screw part 13, a screw thread angle in its top part 14a is set to 60° and a screw thread angle in a base part 14b excluding the top part 14a is set to 45°. Further, the height dimension (m) of the top part 14a is set to 40% of the total height dimension (h) of the screw thread, and the height dimension (n) of the base part 14b is set to 60% of the total height dimension (h) of the screw thread. On the other hand, for the male screw part 21 of the nut 20, the angle of its screw thread 22 is set to 60°.