is connected to the axial center of the back side of the impeller wheel 9 via a hollow connecting shaft 35 having curvic tradename couplings 34 a and 34 b on either axial end thereof the axial center of the back side of the turbine wheel 11 is connected to a collar 36 and serving as an extension of the connecting shaft 35 and fitted with a labyrinth seal 32 c adjacent to a bearing portion on the rear end of the outer shaft 7 via a curvic coupling 34 c the impeller wheel 9 hollow connecting shaft 35 turbine wheel 11 collar 36 and inner race of the bearing 5 r on the rear end of the out shaft 7 are fitted onto the outer shaft 7 in this order and a bearing nut 37 threaded onto the outer shaft 7 applies a prescribed initial tension to the outer shaft 7 behind the impeller wheel 9 is defined an impeller back chamber 39 by a back plate 38 and the output pressure of the high pressure centrifugal compressor hc prevails in the impeller back chamber 39 the hollow connecting shaft 35 is provided with a circumferentially elongated slot 40 to communicate an annular gap 41 defined between the inner circumferential surface of the connecting shaft 35 or the axial center of the impeller wheel 9 and the outer circumferential surface of the outer shaft 7 with the impeller back chamber 39 because the opening provided in the hollow connecting shaft 35 consists of the circumferentially elongated slot 40 the stress acting on the inner periphery of the opening due to the centrifugal force can be minimized and the swirl resulting from the impeller.