An exhaust gas purification system of an internal combustion engine comprising a lean NOx trap catalyst device provided in an exhaust passage of the internal combustion engine and a control device configured to perform a rich control for recovering a NOx occlusion capacity of the lean NOx trap catalyst device, wherein the control device comprises: NOx discharge amount calculating means configured to calculate a NOx discharge amount per unit time discharged from the internal combustion engine; NOx occlusion amount determining means configured to calculate a NOx occlusion amount occluded in the lean NOx trap catalyst device and determining to start the rich control when the NOx occlusion amount becomes equal to or greater than a preset NOx occlusion amount threshold value; NOx conversion efficiency determining means configured to calculate a NOx conversion efficiency purified in the lean NOx trap catalyst device and determining to start the rich control when the NOx conversion efficiency becomes equal to or less than a preset NOx conversion efficiency threshold value; rich-wait-time threshold value setting means configured to set a rich-wait-time threshold value on the basis of the NOx discharge amount per unit time calculated by the NOx discharge amount calculating means; elapsed time determining means configured to determine to start the rich control when an elapsed time, which is an elapsed engine operation time after the rich control is performed on the lean NOx trap catalyst device or an elapsed engine operation time after the engine is started, becomes equal to or greater than the rich-wait-time threshold value set by the rich-wait-time threshold value setting means; and rich control means for starting the rich control by a command to start the rich control, and wherein rich control command means is configured to issue the command to start the rich control on condition that three start determinations of determination of the NOx occlusion amount determining means that the rich control is to be started, the determination of the NOx conversion efficiency determining means that the rich control is to be started and the determination of the elapsed time determining means that the rich control is to be started coincide. NOx discharge amount calculating means configured to calculate a NOx discharge amount per unit time discharged from the internal combustion engine; NOx occlusion amount determining means configured to calculate a NOx occlusion amount occluded in the lean NOx trap catalyst device and determining to start the rich control when the NOx occlusion amount becomes equal to or greater than a preset NOx occlusion amount threshold value; NOx conversion efficiency determining means configured to calculate a NOx conversion efficiency purified in the lean NOx trap catalyst device and determining to start the rich control when the NOx conversion efficiency becomes equal to or less than a preset NOx conversion efficiency threshold value; rich-wait-time threshold value setting means configured to set a rich-wait-time threshold value on the basis of the NOx discharge amount per unit time calculated by the NOx discharge amount calculating means; elapsed time determining means configured to determine to start the rich control when an elapsed time, which is an elapsed engine operation time after the rich control is performed on the lean NOx trap catalyst device or an elapsed engine operation time after the engine is started, becomes equal to or greater than the rich-wait-time threshold value set by the rich-wait-time threshold value setting means; and rich control means for starting the rich control by a command to start the rich control, and