PURPOSE: To reduce stress which acts on a rotor in the direction of the center of its rotation axis during the shrinkage of a sleeve in the case of fitting work, in the rotor outside which the sleeve is fitted by shrinkage fitting.

CONSTITUTION: A cylindrical sleeve 15 is fitted outside a rotor 5b rotating in a magnet motor 5 by shrinkage fitting. A spiral groove 15b is gougedly provided on the peripheral face 15a of the sleeve 15. The rigidity of the sleeve 15 is therefore lowered in the direction of the center of its rotation axis to reduce stress caused in the direction of the center of the rotation axis of the rotor 5b by the shrinkage of the sleeve 15 during shrinkage fitting. In addition to that, at the time of the rotation of the rotor 5b, fluid is circulated between the sleeve 15 and a stator 5b by the pumping action of the spiral groove 15b for cooling the sleeve 15 and the stator 5a.