A steam condenser which condenses steam exhausted from a steam turbine. Heat transfer tubes are arrayed below the steam turbine inside the container. Cooling medium flows inside the heat transfer tubes. The heat transfer tubes extend horizontally, and include at least two upper heat transfer tube groups and at least two lower heat transfer tube groups arranged with a gap between each other. Each heat transfer tube group is constituted by arraying heat transfer tubes like a grid. At a lower part between the lower heat transfer tube groups, a baffle plate which obstructs flow of steam extends horizontally. Between the upper and lower heat transfer tube groups, inter-tube-group inundation prevention plates extend horizontally. In each heat transfer tube group, an enclosure part extends to guide gas from the enclosure part to outside of the container through a gas extraction duct.