A return passage that connects two points of a rolling passage to allow each ball that rolls in the rolling passage to endlessly circulate is formed in a ball screw nut by attaching a circulation member to fitting holes. A protrusion is formed on an outer surface of the circulation member at a position offset from a raceway of the balls that circulate in the return passage. The circulation member is fixed to the ball screw nut when the protrusion contacts an inner periphery of an inner ring of a second bearing.