A ball screw device has a screw shaft that has a screw portion having a spiral-shaped shaft-side ball screw groove on an outer peripheral surface thereof, and a carrier integrally formed with the screw portion and constituting a planetary speed-reducing mechanism; a nut that has a spiral-shaped nut-side bass screw portion on an inner peripheral surface thereof; a plurality of balls arranged between the shaft-side ball screw groove and the nut-side ball screw groove; and a rolling bearing for supporting the carrier which has an outer ring having an outer ring raceway, an inner ring raceway provided on a portion facing the outer ring raceway in a radial direction, and a plurality of rolling bodies arranged between the outer ring raceway and the inner ring raceway, which is directly formed on an outer peripheral surface of the carrier.