An air pollution control system comprising: a boiler for combusting fuel; an air heater for recovering heat of a flue gas supplied from the boiler; a precipitator for removing soot and dust contained in the flue gas from which heat has been recovered; a desulfurization device for removing a sulfur oxide contained in the flue gas, from which soot and dust have been removed, by an absorbent; a dehydrator for removing gypsum from desulfurization waste water discharged from the desulfurization device; a spray drying apparatus (50, 50A, 50B) for dehydrator filtrate from desulfurization waste water, the spray drying apparatus (50, 50A, 50B) comprising: a gas inlet provided in a side wall in the vicinity of a top and upper portion of a body of the spray drying apparatus (50, 50A, 50B), for introducing a flue gas for drying a spray; flow-adjusting plates provided in the body of the spray drying apparatus (50, 50A, 50B), 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 dehydrator filtrate from desulfurization waste water into the flue gas having been changed to the laminar flow; and a joining unit for joining a bottom and lower portion of the body of the spray drying apparatus (50, 50A, 50B) to a main flue gas duct (L 3) for the flue gas so as to discharge spray-dried solids to the main flue gas duct (L 3), the spray drying apparatus (50, 50A, 50B) being joined to the main flue gas duct (L 3) between the air heater and the precipitator for spraying dehydrator filtrate supplied from the dehydrator; and a flue gas introduction line (L 11) branched from a flue gas supply main line (L 2) upstream of the air heater for introducing a part of the flue gas through the gas inlet to the spray drying apparatus (50, 50A, 50B). a boiler for combusting fuel; an air heater for recovering heat of a flue gas supplied from the boiler; a precipitator for removing soot and dust contained in the flue gas from which heat has been recovered; a desulfurization device for removing a sulfur oxide contained in the flue gas, from which soot and dust have been removed, by an absorbent; a dehydrator for removing gypsum from desulfurization waste water discharged from the desulfurization device; a spray drying apparatus (50, 50A, 50B) for dehydrator filtrate from desulfurization waste water, the spray drying apparatus (50, 50A, 50B) comprising: a gas inlet provided in a side wall in the vicinity of a top and upper portion of a body of the spray drying apparatus (50, 50A, 50B), for introducing a flue gas for drying a spray; flow-adjusting plates provided in the body of the spray drying apparatus (50, 50A, 50B), 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 dehydrator filtrate from desulfurization waste water into the flue gas having been changed to the laminar flow; and a joining unit for joining a bottom and lower portion of the body of the spray drying apparatus (50, 50A, 50B) to a main flue gas duct (L 3) for the flue gas so as to discharge spray-dried solids to the main flue gas duct (L 3), a gas inlet provided in a side wall in the vicinity of a top and upper portion of a body of the spray drying apparatus (50, 50A, 50B), for introducing a flue gas for drying a spray; flow-adjusting plates provided in the body of the spray drying apparatus (50, 50A, 50B), 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 dehydrator filtrate from desulfurization waste water into the flue gas having been changed to the laminar flow; and a joining unit for joining a bottom and lower portion of the body of the spray drying apparatus (50, 50A, 50B) to a main flue gas duct (L 3) for the flue gas so as to discharge spray-dried solids to the main flue gas duct (L 3), the spray drying apparatus (50, 50A, 50B) being joined to the main flue gas duct (L 3) between the air heater and the precipitator for spraying dehydrator filtrate supplied from the dehydrator; and a flue gas introduction line (L 11) branched from a flue gas supply main line (L 2) upstream of the air heater for introducing a part of the flue gas through the gas inlet to the spray drying apparatus (50, 50A, 50B).