A feed-water discharge side of a condenser is connected to a feed-water entry side of an flue gas cooler through a bypass line provided with a steam production preventive pump and with an inlet cutoff valve. A feed-water discharge side of the flue gas cooler is connected to the feed-water entry side of the condenser through a steam production preventive water circulation line provided with an outlet cutoff valve. When a boiler feed-water pump is stopped in boiler fuel cutoff, the inlet and outlet cutoff valves are opened and the steam production preventive pump is activated to cause water to flow through the bypass line into the flue gas cooler, is returned through the steam production preventive water circulation line to the condenser and is circulated.