PURPOSE: To reduce leak of compressed gas to the back surface side of an impeller, and reduce friction loss by setting a front part of a rotor shaft to have the same outer diameter as that of the impeller for connecting them to each other without a gap, and supporting it to be capable of floating and rotating by a gas bearing to a housing.

CONSTITUTION: In a centrifugal compressor C, the inside of a housing 1 is parted by a parting wall 2 into a front chamber 3 and a back chamber 4. An impeller 5 is contained in the front chamber 3, a gas entrance 6 is opened at a front part of it, and a compression space 7 is formed around it. In the meanwhile, a motor 8 is contained in the back chamber 4, and it is composed of a stator 9, a rotor 10, and a rotor shaft 11. In this constitution, a front shaft part 12 of the rotor shaft 11 is set to have the same outer diameter as that of the impeller 5 and the rotor 10, and it is connected concentrically to it with its back surface tightly applied to a front end surface of the rotor 10 without a gap. The front shaft part 12 is supported to be capable of floating and rotating to a front shaft receiving hole 14 by a gas bearing 16.