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