To enhance support rigidity of a bearing by minimizing the amount of lubricating medium to be employed and reducing the axial dimension.

The inside wall of a first housing 1 is provided with a low speed ring 32, a low speed shaft 3, a disc-like member 4, and a stepped part 70 swelled stepwise from the inside wall of the first housing 1 corresponding to a shape of the inside wall of a snap ring R and forming a gap with a prescribed interval. The stepped part 70 is, for example, equipped with a first stepped part 71 swelled to the snap ring R and a second stepped part swelled to the disc-like member 4.