PURPOSE: To reduce the internal pressure, thermal shock and piping reactive force at the uncontinuous parts in the outer casing structure by protecting these parts with a covering plate or casing.

CONSTITUTION: Rotary shaft 16 is born through a static pressure bearing 7 by a bearing table 5, on which, together with the impeller casing 20, a covering casing 24 is arranged with a certain spacing against the outer casing 11. The covering casing 24 covers the impeller part 2, forming a high pressure chamber 3. Accordingly, liquid sodium as lubricant for the static pressure bearing, as shown by the arrow, flows through the gap between the covering 24 and outer 11 casings to be then returned to the suction nozzle 1. Thereby the outer casing 11 constituting a pressure resistant boundary is separated from the high pressure chamber 3, so that its inner pressure and thermal shock are reduced and even the piping reactive force can be lessened.