PURPOSE: To achieve the protection of a shaft seal device by providing a partition wall in the gas chamber at the upper part of a casing so as to divide it into upper and lower chambers, and penetrating a main shaft through this partition wall allowing a small clearance.

CONSTITUTION: On the inner surface of the circumferential wall 14c of the upper part of a column 14 on a casing 10, a partition wall 21 dividing the gas chamber 16 into upper and lower chambers 15a, 15b is fixed. A main shaft 17 penetrates a bushing 22, and a small clearance is provided between the main shaft 17 and the bushing 22. On the top wall 14a of the column 14, one end of a gas supply pipe 23 is communicated with the upper chamber 16a, and the air under a prescribed pressure is continuously supplied. Thus, the shaft seal device can be securely protected from the vapor generated from a liquid or the liquid surface.