Abstract: A ball nut 30 has a dowel 60 for circulating balls 50. A cage type retainer 40 is disposed between a screw shaft 20 and the ball nut 30 which engages with the screw shaft 20 through the ball 50. The retainer 40 is provided with a plurality of elongated holes 42 at a peripheral surface thereof to extend to an axial direction thereof so as to separate the balls 50 from one another in a peripheral direction thereof and to hold the balls therein to be free to roll. The distance of a space between the screw shaft 20 and the cage type retainer 40 is set to be smaller than that of a space between the dowel 60 and the cage type retainer 40 n the radial direction of the screw shaft.