A ball screw mechanism is equipped with a ball screw shaft formed from a metallic material, a displacement nut inserted over an outer circumferential side of the ball screw shaft, steel balls installed between a first screw groove formed on the ball screw shaft and a second screw groove formed in the displacement nut, and a pair of first and second return members, through which the steel balls are circulated between one end side and another end side of the displacement nut. The first and second return members include main body parts mounted respectively onto one end and another end of the displacement nut, and cylindrical parts that project outwardly with respect to the main body parts. A return passage, through which the steel balls are circulated, is formed inside of the main body parts and the cylindrical parts.