shaft and the magnetic fluid retainer conversely in case of the magnetic fluid retainer according to the invention the drop in pressure resistance is very small even when the retainer is mounted on a magnetic housing this is because even when the magnetic fluid retainer of the invention is mounted on a magnetic housing the gap space of 0 5 mm or larger which exists between a yoke and the housing increases the reluctance at that portion suppressing the return through the housing of the magnetic flux produced by the magnet consequently it becomes possible to increase relatively the amount of magnetic flux to the magnetic fluid retaining gap portion between the shaft and the magnetic fluid retainer permitting to realize a shaft sealer of excellent characteristics these phenomena are clearly seen in the results of computerized magnetic field analysis shown in fig7 that is in fig7 the magnetic flux 10 produced by the magnet 7a mostly flows toward the shaft 1 through the yoke 7b 1 with a large amount of magnetic flux passing across the magnetic fluid retaining gap portion 5 this owes to the provision of the large magnetic gap which is formed between the yoke 7b 1 of smaller outer diameter and the inner periphery 3b of the stepped portion substantially equal to the outer diameter of the larger yoke 7b 2 the gap increasing the reluctance at that portion to block the flow of magnetic flux shown in fig8 are variations in pressure.