The present invention provides a screw device which takes a heavy load in one direction along the axis of the screw shaft and only a light load in the opposite direction, the screw device capable of providing ease of control of preloading.The screw device of the present invention has a screw shaft 1 with an outer surface having a ball rolling groove 1a for rolling balls 7 and a roller rolling groove 1b for rolling rollers 6 both spirally formed thereon; a ball nut 4 with an inner surface having a ball rolling groove 4a spirally formed thereon facing the ball rolling groove 1a of the screw shaft 1; a roller nut 5 with an inner surface having a roller rolling groove 5a spirally formed thereon facing the roller rolling groove 1b of the screw shaft 1; a plurality of balls 7 arranged between the screw shaft 1 and the ball nut 4; and a plurality of rollers 6 arranged between the screw shaft 1 and the roller nut 5. The ball nut 4 and the roller nut 5 are connected along an axis of the screw shaft 1.