Disclosed is a technology that enables better reduction of NOx stored in an NOx catalyst in an exhaust gas purification system for an internal combustion engine. When NOx stored in an NOx catalyst is to be reduced, the air-fuel ratio of the ambient atmosphere around the NOx catalyst is decreased to a target air-fuel ratio by decreasing the air-fuel ratio of the exhaust gas discharged from the internal combustion engine. In doing so, if the temperature of the NOx catalyst is not lower than a specific temperature, the air-fuel ratio of the ambient atmosphere around the NOx catalyst is decreased to the target air-fuel ratio while bringing the combustion state into low temperature combustion. On the other hand, if the temperature of the NOx catalyst is lower than the specific temperature, the air-fuel ratio of the ambient atmosphere around the NOx catalyst is decreased to the target air-fuel ratio while decreasing the quantity of EGR gas in a cylinder so that the amount of generated smoke is kept within an allowable range.