Abstract: Hydrochloric acid is produced by capturing hydrogen chloride contained in an exhaust gas resulting from a gas-phase chemical reaction process involving a dehydrochlorination reaction, the hydrochloric acid is guided into one or some refinery columns of a plurality of refinery columns arranged in parallel so that iron ions are removed from the hydrochloric acid, in accordance with deterioration of iron ion removal capability in the one or some refinery columns, the removal of the iron ions from the hydrochloric acid is switched to another one or some of the refinery columns, while the hydrochloric acid is guided into the other one or some refinery columns so that the iron ions are removed from the hydrochloric acid, the deteriorated iron ion removal capability is regenerated.