A fluid dynamic bearing mechanism for a motor (1) suitable for use in a hard disk drive and having a compact and thin shape, high bearing rigidity, and high rotating accuracy, and which securely keeps the rotor member (6) in place against shocks, and allows the inspection of lubricant supply amount easily. A fluid dynamic bearing mechanism having a capillary seal part (12) on one end of lubricant supply part formed by a minute gap including dynamic pressure grooves (10) formed on a shaft member (5) or a bearing member (4) is provided. An annular member (13) is fitted on the shaft member at the location corresponding to the capillary seal part, another annular member (14) is fitted on the bearing member at the location corresponding to the capillary seal part, a taper or step (13a, 14a) is formed on the outer peripheral surface of the annular member on the shaft member side and the inner peripheral surface of the annular member on the bearing member side. These inner and outer peripheral surfaces are arranged to be close to and facing with each other so that the rotor member is prevented from disengaging from the bearing member, and the capillary seal part can be formed at the same time. An axial dynamic pressure bearing unit (1) is formed between the annular member on the shaft member side and one end of the bearing member.