A spray drying apparatus includes: a gas inlet provided in a side wall in the vicinity of a top (lid) portion of the spray drying apparatus, for introducing a flue gas for drying a spray of dehydrated filtrate; flow-adjusting plates provided in a body of the spray drying apparatus, for decelerating the introduced flue gas so as to change the flow of the flue gas to a laminar flow; a spray nozzle for spraying the dehydrated filtrate from the desulfurization waste water into the flue gas having been changed to the laminar flow; and a joining unit for joining a bottom portion of the body of the spray drying apparatus to a gas supply line that is a main flue gas duct for the flue gas so as to discharge spray-dried solids to the main flue gas duct.