An apparatus for treating flue gas using a single-stage gas dispersing tray, which removes sulfur dioxide from the flue gas exhausted from a thermal power plant. Oxidation air is injected into the lower portion of an absorption slurry riser so as to decrease the density of a gas-liquid (slurry) mixture rising through the absorption slurry riser, and a difference of densities of absorption slurry between the inside and the outside of an overflow weir serve as a driving force for circulating the absorption slurry positioned on and under the gas dispersing tray so as to maximize the amount of the circulating absorption slurry. Thereby, the apparatus has a high SO.sub.2 removal efficiency even at a low operating pH and a low absorber pressure drop.