PURPOSE: To selectively measure nitric acid and hydrofluoric acid so as to obtain highly reliable data by adopting the neutralization titration and substitution titration methods.

CONSTITUTION: A sample sent from a sample introducing tube 7 is introduced to a reactor 1 by a fixed amount through a weighting tube 10. Then, after a reducing reagent is fed to the reaction container 1 from a reducing reagent tank 6 while water is fed to the container 1 by opening a valve 9a and the sample and water are agitated with an agitator 2, a neutralization titration reagent of a known concentration is fed to the container 1 from a neutralization titration reagent tank 4 and the total acid concentration is measured by neutralization titration. At the time of measuring the concentration of nitric acid, the concentration is found by substitution titration by feeding to the container 1 a nonaqueous solvent, such as ethanol, etc., by opening a valve 8a and a substitution titration reagent from a substitution titration reagent tank 5. Then the concentration of hydrofluoric acid is found from the concentrations of total acid and nitric acid.