An air pollutant control system comprises: a denitration apparatus that reduces nitrogen oxide in flue gas discharged from a coal combustion boiler, and that sprays hydrogen chloride into the gas to oxidize mercury, an air heater that recovers heat in gas, a dust collector that reduces dust in gas, a desulfurization apparatus that reduces sulfur oxide in gas from which the dust has been reduced, a hydrogen chloride vaporizer that evaporates concentrated hydrochloric acid to obtain the hydrogen chloride, and a hydrochloric acid neutralization tank where dilute hydrochloric acid discharged from the hydrogen chloride vaporizer or the concentrated hydrochloric acid is neutralized with an alkali agent. Neutralized chloride is supplied to a fuel feeder, mixed with a fuel, and then burned as a fuel in a boiler to produce hydrogen chloride in flue gas. Together with sprayed hydrogen chloride derived from the hydrogen chloride vaporizer, the mercury is reduced.