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In this study, we focused on the diamagnetism of ester lubricant oil for fluid dynamic bearings (FDBs) and investigated the effect of a magnetic field on oil vapors. We vaporized the lubricant oil and compared the amount of oil adsorbed on disk surfaces through two slits-one with magnets and the other without a magnet-in a preliminary experiment. As a result, we confirmed that it is possible to reduce the amount of oil adsorbed on a disk surface with the aid of a magnetic field. Based on the preliminary experiment, the outflow of the FDB oil from the hard disk drive (HDD) spindle was compared with regard to a narrow clearance with and without the magnetic field gradient of permanent magnets. The amount of outflow was decreased by the magnetic field gradient, and the reduction ratio depended on the environmental temperature. Finally, our results suggest the possibility of a gas seal generated by the magnetic field gradient.