Since it is configured that an female screw groove 2a of a nut 2 extends round an axis of the nut 2 through 360 degrees or less and a sectional shape thereof taken at right angles to the groove contains a circular arc portion 2c of 90 degrees or less and a rectilinear portion 2d which connects to the circular arc portion at a bottom of the groove and extends in parallel with the axis, after molding, a lower mold LM and an upper mold UM can simply be removed from a barrel mold BM in the axial direction, and consequently, the necessity of provision of a parting plane which passes through the thread groove 2a can be obviated, thereby making it possible to easily form the thread groove 2a which is free from a difference in level or a stepped portion.