PURPOSE: To enhance deaeration capacity of the whole equipment by sufficiently performing condensation action of steam contained in fluid sucked by vacuum.

CONSTITUTION: The supply line 2 of raw water is connected to one end of a deaeration module 1 and the discharge line 3 of treated water is connected to the other end thereof. A vacuum deaeration line 5 interposing a water-sealed vacuum pump 4 is connected to the deaeration module 1. Condensation action of steam contained in fluid sucked by vacuum is effectively performed by providing a cooling means 6 for cooling fluid in the vacuum deaeration line in the upstream position of the water-sealed vacuum pump 4 interposed in the vacuum deaeration line.