Abstract: A rotation support device for a pinion shaft is configured such that, together with being able to sufficiently maintain axial rigidity of a rolling bearing on the side opposite from the pinion gear side, it is able to sufficiently reduce the dynamic torque during operation and the starting torque at the start of operation. Together with using a single-row angular contact ball bearing as the rolling bearing on the side opposite from the pinion gear side, the ratio of the radius of curvature Ro of the outer raceway and the diameter Bd of the balls (Ro/Bd) is regulated within the range 0.52<Ro/Bd?0.56, and the contact angle ? is regulated within the range of 35° to 40°.

Abstract: When a minimum thickness h of a part where an outer ring raceway is provided on a middle portion in the axial direction of an outer ring and a diameter Da of each rolling element satisfy the relationship 0.4Da?h 0.8Da, even when the outer ring is fixed in a transmission case of low-rigidity, elastic deformation of the outer ring can be prevented without increasing the thickness of the outer ring. When an outer diameter D of an outer ring, a width W of this outer ring in the axial direction, a minimum thickness h of a part where the outer ring raceway is provided on a middle portion in the axial direction of the outer ring, and a diameter Da of each ball are controlled such that a value K calculated by {(h1.5·W)/(Da1.1·D0.5)} satisfies the relationship 1.20?K?2.00, sufficient rigidity of the outer ring can be maintained, and premature flaking based on elastic deformation of the outer ring can be prevented without needlessly increasing the size of the outer ring.