To improve reliability in a feed water pump, by preventing excessiveness of specifications of an apparatus (such as a pipe and a valve) arranged in a feed water system on a condensate pump outlet side in a once-through heat recovery boiler system.

This once-through heat recovery boiler adopts a once-through system as a high pressure system, and adopts a natural circulating system as a medium-low pressure system, and is constituted so that a first line provided with a condensate-desalting device forms the once-through system, by feeding water, by being divided into the first line provided with the condensate-desalting device 7 and a high pressure feed water pump 20 and a second line provided with a medium-low pressure feed water pump 21, on the outlet side of a condensate pump 6 arranged in an alternating current of condenser 5. The second line is also constituted of two lines respectively provided with a medium pressure feed water pump and a low pressure feed water pump.