When an outer diameter of an outer ring 4 is designated as D, an inner diameter of an inner ring 2 is designated as d, and a pitch circle diameter of respective balls 6 is designated as dm, a relationship of ((D+d)/2×0.85≦dm≦(D+d)/2×0.97 is satisfied. Further, when a diameter of the respective balls 6 is designated as Da, a axial width of a ball bearing is designated as B, a distance between centers of respective adjacent balls 6 in a circumferential direction is designated as L, and a sectional height of the ball bearing calculated by a relationship of H=(D−d)/2 is designated as H, all of relationships of 0.60≦Da/H≦0.75, and 0.58≦Da/B≦0.85, and 1.03Da≦L≦1.25Da are satisfied. Thereby, a structure capable of reducing rotational torque is realized without sacrificing durability.