Abstract: There provides a ceiling surface of a deflector which secures an allowance for engagement between a rolling ball and a retainer. A ball screw device includes a ball screw shaft, a ball nut, a deflector forming a coupling passage, a rolling ball, and a retainer. The coupling passage is defined by an outer circumferential ball rolling groove, a ridge, and a ceiling surface. The ceiling surface has a first rounded portion at which a projected shape of the ceiling surface seen in an axial direction has a first radius of curvature at least such that a constant first clearance is provided between the rolling ball and the ceiling surface, and a second rounded portion at which the projected shape has a second radius of curvature smaller than the first radius of curvature and which is located on a groove bottom side of the outer circumferential ball rolling groove.