PURPOSE: To prevent corrosion of floating rings for a seal and improve seal function.

CONSTITUTION: A casing 5 has an inside and an outside seal labyrinths 6, 7 and also an inside and an outside floating rings 1, 2 held therein by an outside seal housings 3, 4, while seal oil 8 is supplied through an inside and outside gaps 10-1, 10-2 between the surface of a rotor 9 and the inside and outside floating rings 1, 2 and flows to the inside 30 as sour drain 8-1 and outside 31 as sweet drain 8-2. The surfaces of the floating rings 1, 2 facing the rotor 9 have carbon materials 11, 12 adhered thereon and, the carbon material being highly corrosion resistant, oil is kept to a suitable quantity without corrosion resistant ingredients accumulating or lacking in the gaps 10-1, 10-2.