This is a method and apparatus for accurately determining a NOx concentration of a measurement gas that contains H2O and/or CO2, without being affected by a dissociation of H2O and/or CO2. In this method and apparatus, (1) the NOx-measurement gas containing H2O and/or CO2 is introduced to travel through a heated flow channel formed by an oxygen ion conductive solid ceramic electrolyte body that electrically control amount of O2 from the flow channel, (2) while said measurement gas travels, a residual gas including NO and H2O and/or CO2 is formed from the measurement gas, (3) said residual gas is flowed to contact a catalytic electrode to which a negative polarity of a low voltage that does not dissociate H2O and/or CO2 but dissociates NO is applied, so that said electrode dissociates a NO gas of the residual gas into N ions and O ions and flows an electric current through the electrolyte body in proportion to an amount of the O ions dissociated from said residual gas at the electrode, and (4) the NOx concentration in the measurement gas is determined based on the current. The low voltage preferably applied in the above (3) is about 350-450 mV.