The invention relates to a wet process flue gas desulfurization apparatus of tank oxidation system comprising an absorption tower having a slurry containing calcium compound supplied in the bottom tank, a circulation pump for feeding the slurry in the tank into the flue gas inlet in the upper part of the absorption tank to contact with flue gas, and air feed means for feeding air for oxidizing into the tank, thereby absorbing and oxidizing sulfur dioxide in the flue gas by the slurry to obtain gypsum as byproduct, wherein a vacuum type belt filter is disposed along the outer surface of side wall of the absorption tower, and part of the slurry discharged from the circulation pump is led into the belt filter to be dehydrated, thereby separating and recovering gypsum.