A method of correcting NOx sensor includes the steps of preparing a corrective map in advance, finding an existing proportion of NO or NO2 in a mixture of NOx in exhaust gases before coming into an NOx sensor, and correcting an NOx concentration that the NOx sensor detects actually. The corrective map records relationships between temperature physical quantities that are relevant to a temperature of the exhaust gases, oxygen-concentration physical quantities that are relevant to an oxygen concentration in the exhaust gases, and the existing proportion. The existing proportion is found with reference to the corrective map using the temperature physical quantities and oxygen-concentration physical quantities that are detected actually. The NOx concentration is corrected on the basis of not only the existing proportion but also a difference between an NO2 diffusion velocity and an NO diffusion velocity.