To reduce the cost, and to improve the performance by facilitating the control of the pre-load of a tapered roller bearing and a ball bearing.

A pinion shaft 1 is rotatably supported by a tapered roller bearing 12 and a ball bearing 16 on the inner side of an outer ring 6 fixed to a casing 3. An inner ring raceway 10 to constitute the tapered roller bearing 12 is formed directly on an outer circumferential surface of the pinion shaft 1. Because the dimensional errors of components and the assembly errors are not integrated, the pre-load of the tapered roller bearing 12 and the ball bearing 16 can be easily controlled.