A method for measuring nitrogen oxide (NOx) in a gas by decomposing NOx in the gas and measuring an amount of oxygen generated by the decomposition includes: introducing the gas to a first inner space (2) via a first diffusion resistance (1) connected with an external space, setting the oxygen partial pressure in the first inner space to a predetermined low value at which NO is not decomposed substantially by a main pump means (60), introducing the controlled atmosphere of the first inner space into a second inner space (9) connected with the first inner space via a second diffusion resistance (8), and measuring an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the atmosphere. A first oxygen concentration detecting means (12) for detecting the oxygen partial pressure is disposed in the second inner space, and the control voltage applied to the main pump means is adjusted on the basis of the output of the first oxygen concentration detecting means. This has the advantage that the system is much less sensitive to interferrents such as oxygen and steam.