PURPOSE: To prevent a damage of a drain pump by arranging a drain cooler between a drain tank and the drain pump and by suppressioning boiling-up of drain by lowering drain temperature at an inlet of the drain pump.

CONSTITUTION: Drain in a drain tank 6 is water (drain) of temperature of a saturated condition or of a little one than that, that is, 150 to 200°C. On the other hand, feed water flowing into a drain cooler 16 through a feed water by-pass line 18, is the water of temperature lower than 50°C, which is fluid that steam condensed in a condenser 14 and is pressurized by a condensate pump 20. Accordingly, at the drain cooler 16, the drain flowing out from the drain tank 6 is heat-exchanged with low temperature feed water and is enough cooled down and therewith, even if pressure reduction occurs, the drain does not boil up and therefore there is no danger for the drain to give any damage to the pump.