A method of correcting an output of a NOx sensor configured to measure a NOx concentration in a measurement gas, by controlling, to a constant value, an oxygen partial pressure in the measurement gas introduced into a first internal space from a measurement-gas space owing to a pumping action based on a pumping current Ip0 supplied to main oxygen pumping means, introducing a controlled atmosphere in the first internal space into a second internal space for reduction or decomposition of NOx in the atmosphere, and pumping out oxygen generated upon the reduction or the decomposition from the second internal space by measurement oxygen pumping means, so as to measure the NOx concentration in the measurement gas based on a pumping current Ip2 flowing through the measurement oxygen pumping means, the method being characterized by comprising: a first step of obtaining a sensor-variation relational expression (x), wherein: the pumping current Ip0 and the pumping current Ip2 are measured for a number of NOx calibration sensors each calibration sensor using a standard gas, under a reference pressure (Po), (whose NOx concentration is already known, for calculating an Ip2/Ip0 value for each of the calibration sensors; an output change percentage (β) is calculated as a ratio of change of a pumping current flowing through the measurement oxygen pumping means obtained in each of the calibration sensors using the standard gas under a prescribed specific pressure different from the reference pressure, to a pumping current flowing through the measurement oxygen pumping means obtained in each of the calibration sensors using the standard gas under the reference pressure; and the sensor-variation relational expression(x) is obtained from a relationship between the Ip2/Ip0 value and the output change percentage (β) for said each of the calibration sensors; a second step of obtaining an output change percentage β for a subject NOx sensor which is different from the calibration sensors and whose output is to be corrected, by calculating, for the subject NOx sensor, the Ip2/Ip0 value using the standard gas under the reference pressure so as to obtain, from the sensor-variation relational expression(x), the output change percentage β that corresponds to the calculated Ip2/Ip0 value; a third step of calculating a pressure correction coefficient α based on the output change percentage β obtained in the second step, according to the following formula: α ⁢ pʹ - p 0 / pʹ = β / β + 1 wherein po represents the reference pressure and p' represents the specific pressure of the standard gas; and a fourth step of performing output correction on the subject NOx sensor, wherein: a NOx concentration in the measurement gas is measured using the subject NOx sensor to detect a pumping current Ip2(p) flowing through the measurement oxygen pumping means while a pressure p of the measurement gas is detected; and a pumping current Ip2(p 0) flowing through the measurement oxygen pumping means under the reference pressure po is calculated based on the pumping current Ip2(p) and the pressure p, according to the following formula: Ip ⁢ 2 p 0 - Ip ⁢ 2 p / Ip ⁢ 2 p = α ⁢ p 0 - p / p so as to perform the output correction on the subject NOx sensor. a first step of obtaining a sensor-variation relational expression (x), wherein: the pumping current Ip0 and the pumping current Ip2 are measured for a number of NOx calibration sensors each calibration sensor using a standard gas, under a reference pressure (Po), (whose NOx concentration is already known, for calculating an Ip2/Ip0 value for each of the calibration sensors; an output change percentage (β) is calculated as a ratio of change of a pumping current flowing through the measurement oxygen pumping means obtained in each of the calibration sensors using the standard gas under a prescribed specific pressure different from the reference pressure, to a pumping current flowing through the measurement oxygen pumping means obtained in each of the calibration sensors using the standard gas under the reference pressure; and the sensor-variation relational expression(x) is obtained from a relationship between the Ip2/Ip0 value and the output change percentage (β) for said each of the calibration sensors; a second step of obtaining an output change percentage β for a subject NOx sensor which is different from the calibration sensors and whose output is to be corrected, by calculating, for the subject NOx sensor, the Ip2/Ip0 value using the standard gas under the reference pressure so as to obtain, from the sensor-variation relational expression(x), the output change percentage β that corresponds to the calculated Ip2/Ip0 value; a third step of calculating a pressure correction coefficient α based on the output change percentage β obtained in the second step, according to the following formula: α ⁢ pʹ - p 0 / pʹ = β / β + 1 wherein po represents the reference pressure and p' represents the specific pressure of the standard gas; and a fourth step of performing output correction on the subject NOx sensor, wherein: a NOx concentration in the measurement gas is measured using the subject NOx sensor to detect a pumping current Ip2(p) flowing through the measurement oxygen pumping means while a pressure p of the measurement gas is detected; and a pumping current Ip2(p 0) flowing through the measurement oxygen pumping means under the reference pressure po is calculated based on the pumping current Ip2(p) and the pressure p, according to the following formula: Ip ⁢ 2 p 0 - Ip ⁢ 2 p / Ip ⁢ 2 p = α ⁢ p 0 - p / p so as to perform the output correction on the subject NOx sensor.