generated in the low pressure boiler drum 1 is directly sent to a low pressure steam turbine the water which has been deaerated by the condenser 12 is boosted to an operating pressure e g 9 atmg by the low pressure pump 3 thereafter enters a low pressure economizer 5 whence a part of water is introduced into the low pressure boiler drum 1 through the flow control valve 10 and the remaining part passes through a pipeline 30 and is boosted to 65 100 atmg for example to be turned into high pressure feed water in the case of start up of a gas turbine where the exhaust gas 20 is comparatively low in temperature and high in flow rate an amount of heat exchange is so increased in the low pressure economizer and the high pressure economizer that steaming in said economizer is liable to occur in this case the flow control valve 10 and or 11 should be throttled down so that the pressure in the economizer may be maintained at more than the value of a saturated steam pressure in the economizer thus steaming or evaporation in the economizer is suppressed thereby making water flow in the economizer stable while the rest of heated feed water is fed to the high pressure water feed system through the pipeline 30 thereby enabling recovery of the heat effectively in fig1 in addition to the control of the feed.