A spray-drying device includes a spray nozzle that sprays the dehydrated filtrate from the desulfurization wastewater, in a spray-drying device body, an inlet that is provided in the spray-drying device body and introduces flue gas for drying spray liquid, a dry area that is provided in the spray-drying device body and dries the dehydrated filtrate by the flue gas, an outlet that discharged the flue gas contributing to the drying, a plurality of thermometers that are provided in the dry area and measure temperatures of the inside, a determination unit that determines whether or not a spray-drying state of the dehydrated filtrate is satisfactory on the basis of the measurement results of the thermometers, and a control unit that adjusts the flue gas or the dehydrated filtrate when it is determined that the spray-drying is not satisfactory as a result of the determination of the determination unit.