A desulfurization apparatus comprising: a gas inlet for introducing a sulfur dioxide containing gas to be treated into the desulfurization apparatus; a desulfurizing part for bringing the sulfur dioxide containing gas to be treated and a desulfurization absorption liquid, into contact with each other to absorb and eliminate the sulfur dioxide from the gas to be treated; a mist eliminator for eliminating the desulfurization absorption liquid accompanying a desulfurized gas obtained by the elimination of the sulfur dioxide in the desulfurizing part from the desulfurized gas, the mist eliminator being provided between the desulfurizing part and a supercooling part mentioned below; a supercooling part for cooling the desulfurized gas obtained by the elimination of the accompanying liquid in the mist eliminator by spraying the desulfurized gas with cooling liquid to produce condensed water from the desulfurized gas; a chimney tray for accumulating the cooling liquid and the condensed water, the chimney tray being provided between the mist eliminator and the supercooling part; a cooling liquid-circulating line that connects the upper and lower sides of the supercooling part for circulating and supplying the cooling liquid and the condensed water that have accumulated in the chimney tray to the supercooling part to recycle the condensed water obtained in the supercooling part as a cooling liquid for cooling the desulfurized gas in the supercooling part; a condensed water line for supplying the condensed water from the supercooling part to a site to be washed to wash the desulfurization apparatus with the condensed water; and a cooler for cooling the cooling liquid through heat exchange with a coolant to circulate and supply the cooling liquid to the supercooling part for recycling the cooling liquid in the supercooling part, the cooler being provided on the cooling liquid-circulating line, wherein the site to be washed comprises: the gas inlet for introducing the gas to be treated into the desulfurization apparatus; and the mist eliminator, and wherein the condensed water line, branches from the cooling liquid-circulating line, into a first condensed water-recycling line for supplying the condensed water to the gas inlet and a second condensed water-recycling line for supplying the condensed water to the mist eliminator. a gas inlet for introducing a sulfur dioxide containing gas to be treated into the desulfurization apparatus; a desulfurizing part for bringing the sulfur dioxide containing gas to be treated and a desulfurization absorption liquid, into contact with each other to absorb and eliminate the sulfur dioxide from the gas to be treated; a mist eliminator for eliminating the desulfurization absorption liquid accompanying a desulfurized gas obtained by the elimination of the sulfur dioxide in the desulfurizing part from the desulfurized gas, the mist eliminator being provided between the desulfurizing part and a supercooling part mentioned below; a supercooling part for cooling the desulfurized gas obtained by the elimination of the accompanying liquid in the mist eliminator by spraying the desulfurized gas with cooling liquid to produce condensed water from the desulfurized gas; a chimney tray for accumulating the cooling liquid and the condensed water, the chimney tray being provided between the mist eliminator and the supercooling part; a cooling liquid-circulating line that connects the upper and lower sides of the supercooling part for circulating and supplying the cooling liquid and the condensed water that have accumulated in the chimney tray to the supercooling part to recycle the condensed water obtained in the supercooling part as a cooling liquid for cooling the desulfurized gas in the supercooling part; a condensed water line for supplying the condensed water from the supercooling part to a site to be washed to wash the desulfurization apparatus with the condensed water; and a cooler for cooling the cooling liquid through heat exchange with a coolant to circulate and supply the cooling liquid to the supercooling part for recycling the cooling liquid in the supercooling part, the cooler being provided on the cooling liquid-circulating line, wherein the site to be washed comprises: the gas inlet for introducing the gas to be treated into the desulfurization apparatus; and the mist eliminator, and wherein the condensed water line, branches from the cooling liquid-circulating line, into a first condensed water-recycling line for supplying the condensed water to the gas inlet and a second condensed water-recycling line for supplying the condensed water to the mist eliminator.