Since NO₂⁻ often coexists with NO₃⁻ in environment and waste fluids, it is important to evaluate precisely the concentrations of NO₂⁻ and NO₃⁻ simultaneously. A new electrochemical consequtive detection system of NO₂⁻ and NO₃⁻ was constructed. After the concentration of NO₃⁻ in the sample solution was detected using an ion selective electrode (NO₃⁻-ISE), persisting NO₂⁻ in the sample solution was oxidized to NO₃⁻ using carbon felt as a working electrode. Then, the concentration of NO₃⁻ in the sample solution was measured using the NO₃⁻-ISE again, whereas the concentration of NO₂⁻ was determined by comparing the results before and after electrolytic oxidation. On the basis of the Coulomb number and via electrolysis, our investigations detected the sample solution’s concentrations of NO₂⁻ and NO₃⁻ successively, thereby easing the evaluation of the concentration of NO₂⁻ for cases above 10⁻⁵ mol dm⁻³.