When a minimum thickness of a part where an outer ring raceway is provided on a middle portion in the axial direction of an outer ring is h, and a diameter of each rolling element is Da, the relationship 0.4 Da.ltoreq.h 0.8 Da is satisfied. As a result, even when the outer ring 4a is fixed in a transmission case of low-rigidity, elastic deformation of the outer ring can be prevented without needlessly increasing the thickness h of the outer ring.When an outer diameter of an outer ring is D, a width of this outer ring in the axial direction is W, a minimum thickness of a part where the outer ring raceway is provided on a middle portion in the axial direction of the outer ring is h, and a diameter of each ball is Da, the respective dimensions are controlled such that a value K calculated by {(h.sup.1.5W)/(Da.sup.1.1.D.sup.0.5)} satisfies the relationship 1.20.ltoreq.K.ltoreq.2.00. As a result, sufficient rigidity of the outer ring can be maintained, and early exfoliation based on elastic deformation of the outer ring can be prevented without needlessly increasing the size of the outer ring.