PURPOSE: To perfectly prevent the leakage of oil mist, by securing horizontal discs to a main shaft approaching to the bottom of a main bearing cover in a vertical shaft water turbine or a reversible pump-turbine so that the atmospheric air is sucked from between the main bearing cover and the main shaft.

CONSTITUTION: A sliding ring part A of a main shaft 1 slides rotating coming into contact with the internal surface of a bearing alloy 3 formed by lining on a main bearing 2. Lubricating oil, which is stored in an annular chamber B surrounded by an inside oil tank 4, oil tank bottom plate 5 and an outside oil tank 6, lubricates the above described sliding part. Further an annular chamber C, surrounded by a main bearing supporter 7, upper part oil tank 8 and a main bearing cover 9, is provided in the upper of the above described chamber B. In the above constitution, a circumferential groove G is provided in the main shaft 1 approaching to the bottom of the main bearing cover 9. And two- split hollow discs 20, 20, mounting their internal contour sides to the groove G while providing flanges H in the external contour sides, are connected by bolts 21 and nuts 22 to form an integrally built rotary disc.