A thermal power plant 1 including: an exhaust gas system 11; a feed-water system 4; a high-pressure feed-water heater 10 provided to the feed-water system 4; the main economizer 36, which is provided to the secondary side of the high-pressure feed-water heater 10 of the feed-water system 4 and increases the temperature of the feed-water by using the residual heat of the combustion gas from the boiler 2; and a catalytic NOx removal equipment 14, which is provided to the secondary side of the main economizer 36 of the exhaust gas system 11 and the exhaust gas at a required temperature or higher is supplied to; and a sub economizer 40, which is provided between the high-pressure feed-water heater 10 and the main economizer 36 and increases the temperature of water by using the exhaust gas on the secondary side of the NOx removal equipment 14, is provided.