PURPOSE: To lengthen service life and to prevent disturbance of operation by forming the raceway surface such that the contact angle between a plane containing the center of ball and perpendicular to spinning axis and a line connecting the contact point between the ball and the raceway surface and the center of the ball, is large at opposite end portions close to a circulation path when compared with the contact angle at other portions.

CONSTITUTION: A threaded shaft 14 and raceway surfaces 10, 12 of a nut 16 have such cross-section as an arched face having larger curvature than that of a ball 18 is folded symmetrically with respect to the center of raceway truck, where the raceway surface 10 has same shape over the entire length. The raceway surface 12 is shaped such that the contact angle θ2 between a plane S containing the center O of the ball 18 and perpendicular to the spin axis X and a line (l) connecting between a contact point C2 and the center O is larger at the opposite end portions of the raceway surface 12 than at other portions. Since rolling between balls 18 and between the ball 18 and the raceway surfaces 10, 12 can be carried out, shortening of service life due to fatigue breakdown and disturbed operation can be suppressed, and rigidity and backlash can be ensured.