To reduce an equipment cost and running cost by forming a condenser of a front stage as a surface condensation type heat exchanger and forming a condenser of a rear stage as a jet ejector condenser in common use as an ejection and a condenser.

The first condenser of the front stage is formed as the surface condensation type heat exchanger 4 and the second condenser of the rear stage as the jet ejector condenser in common use as an ejector and a condenser constituting part of a momentum moving type vacuum pump which functions even if a condensed water temp. attains 50 to 70°C. As a result, a manufacturing cost is reduced without using two units of heat exchangers. Since a high condensed water temp. suffices, the use of an inexpensive cooling tower making it possible to obtain a large volume of circulating water is eventually made possible and the initial running cost is reduced. Further, the intrusion of air bubbles into the condensed water does not occur and high condensation efficiency is obtd.