An air pollution control system comprises: a carry-in tank truck that carries in a feedstock, i.e., concentrated hydrochloric acid (35% hydrochloric acid); a hydrogen chloride vaporizer that vaporizes the concentrated hydrochloric acid to obtain hydrogen chloride; a liquid level regulator that separates residual hydrogen chloride from extracted dilute hydrochloric acid discharged as a by-product from the hydrogen chloride vaporizer, and that regulates the liquid level of the hydrogen chloride vaporizer; a concentration regulating tank that regulates the hydrochloric acid concentration of extracted dilute hydrochloric acid 25B from which the residual hydrogen chloride has been separated to a predetermined concentration (e.g., 22%); a by-product tank that stores therein dilute hydrochloric acid regulated to have a predetermined concentration; and a carry-out tank truck that carries out the dilute hydrochloric acid with a predetermined concentration discharged from the by-product tank.