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A nitrogen oxide concentration detector has a first measurement chamber 2 into which is introduced a measurement gas via a first diffusion resistance 1; an oxygen concentration detection electrode 7a for measuring the oxygen concentration in the measurement gas in said first measurement chamber 1; a

A nitrogen oxide concentration detector has a first measurement chamber 2 into which is introduced a measurement gas via a first diffusion resistance 1; an oxygen concentration detection electrode 7a for measuring the oxygen concentration in the measurement gas in said first measurement chamber 1; a first oxygen ion pump cell 6 for pumping out oxygen in the measurement gas from said first measurement chamber 2 based on the potential of said oxygen concentration detection electrode 7a; a second measurement chamber 8 into which the gas is introduced from said first measurement chamber 2 via a second diffusion resistance 3; and a second oxygen ion pump cell 8 having a pair of electrodes 8a,8b across which a voltage is applied to decompose NOx in the second measurement chamber 4 to pump out dissociated oxygen to cause a circuit Ip2 corresponding to the NOx concentration to flow in the second oxygen ion pump cell 8. Variation of NOx concentration is a function of variation of Ip2. The concentration obtained based on the Ip2 is corrected responsive to the oxygen concentration in the measurement gas. Particularly, a coefficient of the variation of the Ip2, gain, in said function is corrected responsive to the oxygen concentration in the measurement gas.

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1. A method for measuring the nitrogen oxide concentration using a nitrogen oxide concentration detector having:a first measurement chamber into which is introduced an exhaust gas via a first diffusion resistance; an oxygen concentration detection electrode for measuring the oxygen concentration in

1. A method for measuring the nitrogen oxide concentration using a nitrogen oxide concentration detector having:a first measurement chamber into which is introduced an exhaust gas via a first diffusion resistance; an oxygen concentration detection electrode for measuring the oxygen concentration in the exhaust gas in said first measurement chamber; a first oxygen ion pump cell for pumping oxygen contained in the exhaust gas from said first measurement chamber to outside and/or from outside to inside of said first measurement chamber based on the potential of said oxygen concentration detection electrode; a second measurement chamber into which the gas is introduced from said first measurement chamber via a second diffusion resistance; and a second oxygen ion pump cell having a pair of electrodes across which a voltage is applied to decompose the nitrogen oxide in said second measurement chamber to pump out dissociated oxygen to cause a current corresponding to the nitrogen oxide concentration, referred to as second oxygen pump current, to flow in the second oxygen ion pump cell: the method comprising: pumping out oxygen from the first measurement chamber with said first oxygen ion pump cell to an extent that can allow some nitrogen oxide to decompose into oxygen in said exhaust gas; and correcting the nitrogen oxide concentration obtained based on said oxygen pump current in response to the oxygen concentration in the exhaust gas.