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, controlling an oxygen partial pressure in an atmosphere in the first inner space to be a predetermined low value at which NO is not decomposed substantially by a main pump means (60), introducing a 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 converting the atmosphere into an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the atmosphere by an electric signal conversion means. A first oxygen concentration detecting means (12) is disposed for detecting an oxygen partial pressure in the second inner space, and a control voltage to be applied to the main pump means is adjusted on the basis of an output of the first oxygen concentration detecting means.