A thermal power plant comprising: a boiler configured to generate steam by increasing a temperature of feed-water by using heat obtained by combusting a fuel; a steam system having a high-pressure steam turbine, a reheater, and an intermediate-pressure steam turbine; an exhaust gas system configured to circulate a combustion gas, which is exhausted from the boiler after combusting the fuel, as an exhaust gas; a feed-water system configured to supply water to the boiler; and a feed-water heater that is provided to the feed-water system and is configured to increase the temperature of the feed-water by using extraction steam; a main economizer, which is provided to a secondary side of the feed-water from the feed-water heater of the feed-water system and is configured to increase the temperature of the feed-water by using residual heat of the combustion gas; and a catalytic NOx removal equipment, which is provided to a secondary side of the exhaust gas from the main economizer of the exhaust gas system and the exhaust gas at 300 oC or higher is configured to be supplied to, wherein a sub economizer, which is configured to increase the temperature of the feed-water by using the exhaust gas on a secondary side of the NOx removal equipment, is provided between the feed-water heater and the main economizer of the feed-water system, the feed-water introduced from the feed-water system to the boiler becomes superheated steam, the superheated steam is introduced into the high-pressure steam turbine and re-introduced in the boiler after being introduced into the high-pressure steam turbine, the steam, which is exhausted from the high-pressure steam turbine and re-introduced in the boiler, is introduced in the intermediate-pressure steam turbine after being reheated in the reheater, the feed-water heater comprises a first feed-water heater and a second feed-water heater in an order from an upstream of the feed-water, a first extraction system, in which extracted steam from the intermediate-pressure steam turbine flows, is provided between the intermediate-pressure steam turbine and the first feed-water heater, and a second extraction system, in which extracted steam from the high-pressure steam turbine to the reheater flows, is provided between the high-pressure steam turbine and the second feed-water heater; characterized in that the sub economizer comprises an exhaust gas temperature control device that is configured to control a temperature of the exhaust gas on a secondary side of the sub economizer, and the exhaust gas temperature control device comprises: a gas duct provided from a primary side to a secondary side of the exhaust gas from the sub economizer of the exhaust gas system; and a gas amount control damper provided to the gas duct. a boiler configured to generate steam by increasing a temperature of feed-water by using heat obtained by combusting a fuel; a steam system having a high-pressure steam turbine, a reheater, and an intermediate-pressure steam turbine; an exhaust gas system configured to circulate a combustion gas, which is exhausted from the boiler after combusting the fuel, as an exhaust gas; a feed-water system configured to supply water to the boiler; and a feed-water heater that is provided to the feed-water system and is configured to increase the temperature of the feed-water by using extraction steam; a main economizer, which is provided to a secondary side of the feed-water from the feed-water heater of the feed-water system and is configured to increase the temperature of the feed-water by using residual heat of the combustion gas; and a catalytic NOx removal equipment, which is provided to a secondary side of the exhaust gas from the main economizer of the exhaust gas system and the exhaust gas at 300 oC or higher is configured to be supplied to, wherein a sub economizer, which is configured to increase the temperature of the feed-water by using the exhaust gas on a secondary side of the NOx removal equipment, is provided between the feed-water heater and the main economizer of the feed-water system, the feed-water introduced from the feed-water system to the boiler becomes superheated steam, the superheated steam is introduced into the high-pressure steam turbine and re-introduced in the boiler after being introduced into the high-pressure steam turbine, the steam, which is exhausted from the high-pressure steam turbine and re-introduced in the boiler, is introduced in the intermediate-pressure steam turbine after being reheated in the reheater, the feed-water heater comprises a first feed-water heater and a second feed-water heater in an order from an upstream of the feed-water, a first extraction system, in which extracted steam from the intermediate-pressure steam turbine flows, is provided between the intermediate-pressure steam turbine and the first feed-water heater, and a second extraction system, in which extracted steam from the high-pressure steam turbine to the reheater flows, is provided between the high-pressure steam turbine and the second feed-water heater; characterized in that the sub economizer comprises an exhaust gas temperature control device that is configured to control a temperature of the exhaust gas on a secondary side of the sub economizer, and the exhaust gas temperature control device comprises: a gas duct provided from a primary side to a secondary side of the exhaust gas from the sub economizer of the exhaust gas system; and a gas amount control damper provided to the gas duct.