a container having at least two side walls, configured to let the steam flow down therein from the steam turbine;

plural heat transfer tubes arrayed below the steam turbine in the container, letting a cooling medium flow inside, contacting the steam flowing down to condense the steam, extending horizontally, and grouped into at least two upper heat transfer tube groups and at least two lower heat transfer tube groups, the two upper heat transfer tube groups being arranged horizontally with a gap therebetween, the two lower heat transfer tube groups being arranged horizontally with a gap therebetween below the two upper tube groups, and each of the heat transfer tube groups including the plural heat transfer tubes arrayed in a grid;

plural tube plates supporting the plural heat transfer tubes;

a baffle plate provided at a lower position between the lower heat transfer tube groups and extending in a horizontal direction, to obstruct a flow of steam;

an inter-tube-group inundation prevention plate disposed at a position between the upper heat transfer tube groups and the lower heat transfer tube groups opposed vertically to each other, the plate extending in a horizontal direction to guide condensed water flowing down from upside, in a horizontal direction;

an enclosure part disposed in each of the heat transfer tube groups, the enclosure part having top plate and two side plates, the top plate being disposed so as to extend substantially horizontally in parallel with the heat transfer tubes and having a gas extraction hole, the two side plates extending downward from the top plate and also extending in parallel with the heat transfer tubes with a space between each other, with the gas extraction hole and the plural heat transfer tubes interposed therebetween; and

a gas extraction duct connected to the gas extraction hole to guide gas from the enclosure part to outside of the container.