A magnetic pre-pressurizing section for pre-pressurizing the inner ring of the bearing in the axial direction by a difference of magnetic forces of permanent magnets of the two rotors is arranged. The magnetic pre-pressurizing section is constructed by a difference of a magnetizing amount for forming permanent magnets 8b, 9b arranged in the respective rotors. The magnetic pre-pressurizing section is also constructed by a difference of distances of an air gap L1 of a stator 2 and a permanent magnet 8b1, and an air gap L2 of the stator 2 and a permanent magnet 9b1. The magnetic pre-pressurizing section is also constructed by a difference of thicknesses of the rotating axis direction of a permanent magnet 8b1 and a permanent magnet 9b2 arranged in the respective rotors.