A ball screw is provided in which an outer circumferential formation is formed at a portion facing a ball circulating passage out of outer circumferential surface of a nut. The ball screw includes: a screw shaft having on its outer circumferential surface a screw groove; a nut having on its inner circumferential surface a screw groove facing the screw groove; a plurality of balls rollably loaded in a spiral ball rolling passage formed by both screw grooves; and a ball circulating passage to return the balls from a start point to an end point of the ball rolling passage for recirculation. The ball circulating passage is a concaved groove formed by concaving a groove on a part of the cylindrical inner circumferential surface of the nut by plastic working. Then, a flange is integrally provided at a portion facing the ball circulating passage and the screw groove.

A motion guide device includes a ball nut attached onto a shaft and having helical grooves for receiving ball bearing elements, a ball circulating member is engaged with the ball nut and includes a ball circulating channel communicative with the helical groove of the ball nut, and includes an intermediate segment and two end segments, a ball coupler includes two flexible coupling members for engaging with the ball bearing elements, and the ball circulating member includes two opposite helical guide pathways for receiving the flexible coupling members of the ball coupler, and curved relative to the ball circulating member for smoothly guiding the flexible coupling members to move through the ball circulating member.