An acid gas recovery method includes removing acid gas from a target gas by bringing the target gas containing the acid gas into gas-liquid contact with an absorbent amine solution and causing the absorbent amine solution to absorb the acid gas; regenerating the absorbent amine solution by releasing the acid gas from the absorbent amine solution that has absorbed the acid gas; causing a chelate resin to adsorb iron ions from the absorbent amine solution by causing the absorbent amine solution to pass through the chelate resin; causing a regenerant solution to pass through a chelate resin having iron ions adsorbed thereon; regenerating the chelate resin and obtaining a regenerant solution containing the iron ions and quantitatively measuring the iron ions in the regenerant solution containing the iron ions and calculating a concentration of iron ions in the absorbent amine solution.