An air pollution control system 10A includes a boiler 11 that burns fuel, an air heater 13 that recovers heat of flue gas 17 from the boiler 11, and a desulfurizer 15 that reduces sulfur oxides contained in the flue gas 17 after heat recovery by an absorbent, and waste-water supplying units P0 to P5 that supply desulfurized waste water 28 discharged from the desulfurizer 15 to at least one of a path for supplying fuel to the boiler 11, inside of a furnace of the boiler 11, and the inside of a flue gas duct between the boiler 11 and the air heater 13 are installed. With this configuration, an amount of desulfurized waste water to be returned into the flue gas duct per unit time can be increased as compared to conventional systems, without increasing the size of the entire air pollution control system.