It is possible to decrease the amount of NOx emitted into the air. The amount of ammonia adsorbed on an SCR catalyst is calculated by subtracting the amount of ammonium nitrate produced on the SCR catalyst from the amount of ammonia adsorbed on the SCR catalyst at the time of starting of an internal combustion engine, and the amount of NO2 flowing into the SCR catalyst is decreased when an NOx purification rate estimated from the amount of ammonia adsorbed on the SCR catalyst is less than a threshold value compared to when the NOx purification rate is equal to or greater than the threshold value.