A wet flue gas desulfurization apparatus includes an absorption tank in which an absorption solution containing an alkaline absorbent is stored; an absorption tower extending upward from the absorption tank; a flue gas inlet duct communicating with a first portion of the absorption tower; a flue gas outlet duct communicating with a second portion of the absorption tower; a spray unit to spray the absorption solution inside the absorption tower; a circulation pump to pump the absorption solution from the absorption tank and to supply the absorption solution to the spray unit; a byproduct discharging unit to discharge a byproduct downward from the absorption tank; and an oxygen supply pipe through which an oxygen-containing gas is supplied to a target point in the absorption tank, the target point being adjustable along a vertical direction or a horizontal direction, enabling adjustment of an internal position to which the gas is supplied.