PURPOSE: To prevent the overheat or abrasion, the loss of shaft sealing function, and the generation of abnormal vibration caused by imperfect lubrication, based on the deformation of a seal casing following the increase of the capacity of the apparatus and the contact of the seal ring with the shaft following the increase of the number of starting and stopping, in the shaft sealing device of an electric rotating machine which uses a hydrogen gas, etc., as a cooling medium.

CONSTITUTION: The title shaft sealing device for an electric rotating machine concerns a device which performs sealing by an oil film, supplying oil into a gap 7 formed between the inside periphery of a seal ring 4 put in a seal casing 3 and the periphery of a shaft 5 with pressure a little higher than the inside pressure of the machine, and the length of the gap 7 below the radial end surfaces of the inside peripheral surface of the seal ring 4 is made larger than that in the center part.