PURPOSE: To ensure an amount of gas required for the operation of a gas extractor by a method wherein a changeover operation of suction of the atmospheric air to recirculation of a portion of non-condensing gas to a condenser is carried out at the start-up time and during loading operation of a steam turbine.

CONSTITUTION: At the start-up time of a steam turbine 1, a start-up ejector 4 is operated to increase the vacuum pressure of a condenser 5, and air suction of a gas extractor 3 which is coaxially connected to the steam turbine 1 is controlled. And, when the load of the steam turbine 1 increases, an air suction valve is closed in response to a loading signal and a portion of delivered gas from the gas extractor 3 is recirculated to the condenser 5 via a gas recirculation pipe 10 and a gas recirculation control valve 17. Thereby, an amount of gas required for the operation of the gas extractor 3 can be ensured.