Patent ID: 8671547

Claim:
A fastening member having an upper portion on a thread top portion side and a lower portion on a thread root side in at least a part of a thread, the upper portion and the lower portion having following constructions, wherein a pressure-side flank surface and a play-side flank surface are formed on the upper portion, a pressure-side side surface and a play-side side surface are formed on the lower portion, the pressure-side side surface is formed to be continuous from a lower end of the pressure-side flank surface, a shape of the pressure-side side surface in a cross-section including an axial line of the fastening member is a curve line shape that curves inward from an extended line of the pressure-side flank surface or a shape provided by combining a straight line shape positioned inside the extended line of the pressure-side flank surface and a curve line shape that curves inward from the extended line, and the lower end of the pressure-side flank surface is farthest from a pressure-side flank surface of a reference thread profile in the pressure-side flank surface and protrudes in a pressure-side direction.