An object of the present invention is to provide a technology that enables to enhance the effect of decreasing the NOx emission achieved by an exhaust gas purification system as a whole while favorably achieving both reduction in the amount of NOx generated in an internal combustion engine by means of EGR and reduction reaction of NOx in an NOx catalyst even when a reduction process for the NOx catalyst is performed while EGR is being performed. In the invention, in the case where addition of fuel into an exhaust pipe from a fuel addition valve 28 is performed to carry out the reduction process for the NOx catalyst 20 when EGR is being performed mainly by means of a low pressure EGR passage 23, the amount of the exhaust gas passing through the NOx catalyst 20 is decreased by decreasing the amount of the exhaust gas recirculated by the low pressure EGR passage 23.