the shaft 4 is fixed to the annular cutout 4 b an annular depression 2 d corresponding to the ring shaped member 26 is formed in the inner circumferential face of the cylindrical through hole 2 b the meshing of the ring shaped member 26 and the annular depression 2 d forms a collar configuration to keep the shaft 4 from slipping out in response to excitation of the stator 8 the rotor hub 2 starts to rotate with respect to the shaft 4 and the bracket 6 according to the foregoing configuration when the rotor hub 2 begins to rotate the lubricant 8 retained in the thrust bearing portion 14 develops thrust load supporting pressure by the action of the thrust dynamic pressure generating grooves 11 with the rotation of the rotor hub 2 likewise the lubricant 8 retained in the radial bearing portion 12 develops radial load supporting pressure by the action of the radial dynamic pressure generating grooves 10 with the rotation of the rotor hub 2 therein the magnetic centers of the rotor magnet 16 and the stator 18 are not coincident and are displaced in the axial direction of the shaft 4 the axial displacement of.