To prevent the occurrence of cavitation by feeding the drain discharged from an attemperator to the attemperator drain suction port of the ejector that has been fitted to the suction side of a water-sealed vacuum pump through an attemperator drain pipe.

To reduce the pressure inside a vacuum distiller or vacuum dryer, depressurized/attemperated steam is fed to the jacket for heating it. The steam after having done work is drained and discharged format the jacket. The drain is forcibly fed into an ejector 30 through a jacket drain suction port 2, and is fed to a water-sealed vacuum pump through 5 a throat and diffuser 6, via a discharge port 7 and a check valve 31. As the steam to be used for a vacuum distiller/vacuum dryer, dipressurized steam is vacuumed, and by adding it sprayed cold water, depresurized/attemperated steam is prepared. The drain is introduced to the attemperator drain suction port 3 of the ejector 30 through a steam trap 14. Thus, the cavitation of the water-sealed vacuum pump can be prevented, and the abnormal damage on the surface due to the cavitation between the casing and the impeller can be eliminated.