A ball screw apparatus which can suppress retaining pieces from contacting a
wall
of a ball rolling path or ball circulating path, and suppress the retaining pieces
between the balls from falling down, so that such a ball screw apparatus can be
improved in noise reduction, operability and endurance. The retaining piece is
rounded at an edge portion opposite to the ball, such that a radius of curvature
of the edge portion is defined to be 0.05 mm to 1 mm, and the outer diameter of
the retaining piece is in a range of 0.60 times or more to 0.85 times or less of
the diameter of the ball.