A centrifugal compressor comprising: a compression mechanism that compresses refrigerant when rotated; a shaft rotatable about a rotation axis and attached to the compression mechanism to rotate a part of the compression mechanism, the shaft having a first portion and a second portion having a smaller diameter than the first portion; a motor arranged to rotate the shaft, the motor including a rotor mounted on the shaft and a stator disposed radially outwardly of the rotor to form a gap between the rotor and the stator; the rotor being slid onto the second portion so as to contact the first portion, and a cooling medium delivery structure including an inlet conduit located to supply a cooling medium to the motor and an outlet conduit located to discharge the cooling medium from the motor, the shaft having an external shape different than an internal shape of the rotor to form at least one axial passageway between the shaft and the rotor along an axial length of the shaft at least as long as an axial length of the rotor, the inlet conduit being located to supply the cooling medium through the gap and the at least one axial passageway to cool the rotor, and the outlet conduit being located to discharge the cooling medium from the gap and the at least one axial passageway, the at least one axial passageway extending along the first and second portions (70, 72) of the shaft, and the first portion being arranged axially adjacent the second portion so that the at least one axial passageway extends in series along the first portion and the second portion of the shaft. a compression mechanism that compresses refrigerant when rotated; a shaft rotatable about a rotation axis and attached to the compression mechanism to rotate a part of the compression mechanism, the shaft having a first portion and a second portion having a smaller diameter than the first portion; a motor arranged to rotate the shaft, the motor including a rotor mounted on the shaft and a stator disposed radially outwardly of the rotor to form a gap between the rotor and the stator; the rotor being slid onto the second portion so as to contact the first portion, and a cooling medium delivery structure including an inlet conduit located to supply a cooling medium to the motor and an outlet conduit located to discharge the cooling medium from the motor, the shaft having an external shape different than an internal shape of the rotor to form at least one axial passageway between the shaft and the rotor along an axial length of the shaft at least as long as an axial length of the rotor, the inlet conduit being located to supply the cooling medium through the gap and the at least one axial passageway to cool the rotor, and the outlet conduit being located to discharge the cooling medium from the gap and the at least one axial passageway, a motor arranged to rotate the shaft, the motor including a rotor mounted on the shaft and a stator disposed radially outwardly of the rotor to form a gap between the rotor and the stator; the rotor being slid onto the second portion so as to contact the first portion, and a cooling medium delivery structure including an inlet conduit located to supply a cooling medium to the motor and an outlet conduit located to discharge the cooling medium from the motor, the shaft having an external shape different than an internal shape of the rotor to form at least one axial passageway between the shaft and the rotor along an axial length of the shaft at least as long as an axial length of the rotor, the inlet conduit being located to supply the cooling medium through the gap and the at least one axial passageway to cool the rotor, and the outlet conduit being located to discharge the cooling medium from the gap and the at least one axial passageway, the at least one axial passageway extending along the first and second portions (70, 72) of the shaft, and the first portion being arranged axially adjacent the second portion so that the at least one axial passageway extends in series along the first portion and the second portion of the shaft.