For the purposes of solving a problem due to a fly ash and HF in an exhaust gas, maintaining the activity of calcium carbonate as an absorbent, and recovering a high-purity gypsum, there is here provided a method for desulfurizing an exhaust smoke which comprises the steps of leading the exhaust gas containing the fly ash to a gas absorbing step, removing SO.sub.2 and the fly ash from the exhaust gas by the use of an absorbing solution slurry containing calcium carbonate as an absorbent in the gas absorbing step, and blowing air into the absorbing solution slurry circulating through the gas absorbing step to oxidize it and to thereby produce the gypsum, the aforesaid method being characterized by drawing a portion of the absorbing solution slurry therefrom, leading it to a magnetic field to separate and recover the fly ash from the absorbing solution slurry, and then returning, to the gas absorbing step, a portion or all of the absorbing solution slurry from which the fly ash has been separated.