A method for wet flue-gas desulfurization in which an exhaust gas discharged from a combustion device including a boiler is introduced into a desulfurization/absorption tower, and a slurry-like absorption liquid prepared by mixing crushed limestone and water into the desulfurization/absorption tower is sprayed from spray nozzles to bring it into contact with the exhaust gas, thus to absorb and remove sulfur oxide in the exhaust gas into the absorption liquid, and the fallen absorption liquid is stored in a circulation tank, while circulating the absorption liquid in the circulation tank and supplying to the spray nozzles so as to repeatedly use the same, a part of the absorption liquid is extracted from the circulation tank and is subjected to solid-liquid separation, thus to recover gypsum generated in the absorption liquid by a desulfurization reaction due to limestone, the method for wet flue-gas desulfurization characterized in that it comprises: supplying powdered activated carbon to the desulfurization/absorption tower to adsorb and remove mercury in the absorption liquid; extracting a part of the absorption liquid from the circulation tank and supplying it to a foam type flotation device which generates a large amount of fine bubbles; recovering a floating foam bubble layer formed on a liquid surface of the absorption liquid of the foam type flotation device to return a part thereof to the desulfurization/absorption tower, and disposing the remaining, as well as, dehydrating the absorption liquid extracted from a lower portion of the foam type flotation device to recover the gypsum; flowing a part of the absorption liquid after recovering the gypsum into a drainage system and circulating the other part to the circulation tank; and adjusting a drainage volume of the absorption liquid flowing in the drainage system after recovering the gypsum, adjusting a circulation amount of the absorption liquid after recovering the gypsum to the circulation tank, and/or adjusting an addition amount of a water-soluble additive containing calcium (Ca) and/or magnesium (Mg) to the absorption liquid in the foam type flotation device, so that physical property values of the foam bubble layer of the foam type flotation device are in a predetermined range. supplying powdered activated carbon to the desulfurization/absorption tower to adsorb and remove mercury in the absorption liquid; extracting a part of the absorption liquid from the circulation tank and supplying it to a foam type flotation device which generates a large amount of fine bubbles; recovering a floating foam bubble layer formed on a liquid surface of the absorption liquid of the foam type flotation device to return a part thereof to the desulfurization/absorption tower, and disposing the remaining, as well as, dehydrating the absorption liquid extracted from a lower portion of the foam type flotation device to recover the gypsum; flowing a part of the absorption liquid after recovering the gypsum into a drainage system and circulating the other part to the circulation tank; and adjusting a drainage volume of the absorption liquid flowing in the drainage system after recovering the gypsum, adjusting a circulation amount of the absorption liquid after recovering the gypsum to the circulation tank, and/or adjusting an addition amount of a water-soluble additive containing calcium (Ca) and/or magnesium (Mg) to the absorption liquid in the foam type flotation device, so that physical property values of the foam bubble layer of the foam type flotation device are in a predetermined range.