A first centrifugal impeller and a second centrifugal impeller are arranged in such an orientation that back sides of the first centrifugal impeller and the second centrifugal impeller face to each other. Bearings 50 are cylindrical roller bearings 51, 52 and a thrust bearing 53. The cylindrical roller bearings are arranged at two axially spaced supporting positions respectively, and support a radial load applied to the rotating shaft 28. The thrust bearing supports a thrust load applied to the rotating shaft 28. One bearing supporting one supporting position of the two bearings supporting two supporting positions is arranged between the first centrifugal impeller and the second centrifugal impeller.