An air pollution control apparatus includes at least one denitration catalyst layer for reducing the amounts of nitrogen oxides in flue gas from a boiler and oxidizing mercury with hydrogen chloride sprayed into the flue gas. Spraying pipe headers 51 are disposed in a flue gas duct 19. The spraying pipe headers 51 are inserted into the flue gas duct 19 and arranged in a direction orthogonal to the direction of a gas flow in the flue gas duct 19. At least four spray nozzles 52-1 to 52-4 are disposed at intervals on the spraying pipe header 51 to form a vertical vortex flow 53 in the gas flow direction. A swirling diffuser plate is disposed on an opening side of the spray nozzles to form the vertical vortex flow in the gas flow direction, wherein the swirling diffuser plate has a flat shape at one side parallel to the spraying pipe header and a wavy shape corresponding to the nozzle intervals on the header at the other side. The diffusion of hydrogen chloride is thereby facilitated in a rapid manner. Therefore, the number of the nozzles can be reduced, and the uniformity of the concentration of hydrogen chloride at the location of the catalyst can be ensured even when the spacing between the nozzles is increased.