A method for treating exhaust gas is provided, which includes measuring SO3 concentration in the exhaust gas which contains at least SO2 and SO3 by using an SO3 gas analyzer; injecting fine particles into the exhaust gas after the step of measuring; cooling the exhaust gas after the step of injection, by recovering heat from the exhaust gas; collecting fly ash from the cooled exhaust gas; and removing at least SO2 from the exhaust gas after the step of collecting by absorbing the SO2 with an absorbing solution. In addition, an apparatus for treating exhaust gas is provided, which includes an SO3 gas analyzer; a fine particles injector; a heat exchanger; an electrostatic precipitator; an absorption tower; and a fine particle feed controller.