To securely discharge start blow water and prevent dew formation at an inlet of an economizer.

In a once-through type exhaust heat recovery boiler, a heat exchanger 27 is provided in the middle of start blow water piping for circulating inner fluid extracted from the lower part of a steam separator 22 provided at the inlet of a superheater 23 to a condenser 5. The heat exchanger 27 includes: cooling water piping 28 extracting and supplying a part of the inner fluid from the downstream side of a low pressure water supply pump 8; and return piping returning the inner fluid heated by heat exchange with the start blow water by the heat exchanger 27 to the upstream side of the low pressure water supply pump 8 to recirculate the inner fluid. Thus, this can suppress flushing within the start blow piping 25, reduce specifications of the start blow piping 25 and a start blow valve 26 and suppress corrosion due to dew formation at the inlet of the economizer 9.