The present invention is concerned with a method for desulfurizing smoke which includes an absorbing procedure of cleaning a combustion exhaust gas with a limestone slurry; an oxidizing procedure of blowing air into the slurry coming from the absorbing process to oxidize calcium sulfite in the slurry to gypsum; a circulating process of circulating, through the absorbing procedure, the slurry coming from the oxidizing procedure; and a separating procedure of drawing a part of the slurry from the circulating procedure, separating and recovering gypsum therefrom, and returning the remaining slurry to the circulating system. The method is characterized in that a separator for separating the fed slurry into an over flow and an under flow is used, and the fed slurry is irradiated with ultrasonic waves to accelerate the separation of gypsum particles from limestone particles, whereby the limestone particles and the gypsum particles are transferred to the over flow and the under flow, respectively.