Disclosed is a method for processing an exhaust gas containing an alkaline substance, the exhaust gas being emitted when activating a carbon material with an alkaline substance (1). The method includes the steps of preparing a humidified gas (2), which exhibits a dew point of 25 °C or more, by mixing an inert gas with water vapor (3), generating a hydroxide of metallic alkali by mixing the humidified gas with the exhaust gas; and trapping the generated hydroxide out of the mixture gas of the humidified gas and the exhaust gas (4).