PURPOSE: To enable to efficiently operate a steam turbine by suppressing the temperature rise of coolant to 7°C or lower, and regulating the degree of vacuum of a condenser without increasing dissolved oxygen in the condensed water.

CONSTITUTION: A condenser 21 has condensation chambers 23, 24, 25 partitioned in a body via cooling tube supporting plates 7 and condensed water surface 22 to communicate with the exit of a steam turbine at the upper opening end, and shielding plates 27 provided in the respective condensation chambers, and the plates 27 perform a role of straightening the turbine exhaust steam by opening the opening ends of the chambers 31, 32, 33, 34. When the degree of vacuum is necessary to be regulated, the exhausted steam is not, when the plates 27 of the chambers 31, 32, 34 are, for example, closed, introduced into the condensation chambers. Thus, the cooling surfaces of cooling tube 6 disposed in this area do not perform their function. Accordingly, the degree of vacuum of the condenser 21 can be regulated by arbitrarily selecting the number of the closing plates 27. On the other hand, since all the coolant contributes to the cooling of the exhausted steam, it can be always suppressed to temperature rise by 7°C or lower.