A control system of an internal combustion engine, the engine comprising an upstream side catalyst which is provided in an exhaust passage (19, 22) of the internal combustion engine, and a downstream side catalyst which is provided in said exhaust passage (19, 22) at a downstream side, in the direction of flow of exhaust, from said upstream side catalyst, said control system comprising: a downstream side air-fuel ratio detecting means which is provide in said exhaust passage (19, 22) between said upstream side catalyst and said downstream side catalyst; and an inflow air-fuel ratio control device which controls an air-fuel ratio of exhaust gas flowing into said upstream side catalyst so that said air-fuel ratio of the exhaust gas becomes a target air-fuel ratio; said control system characterized by comprising a storage amount estimating means for estimating an oxygen storage amount of said downstream side catalyst; a normal period lean control means for setting said target air-fuel ratio of exhaust gas flowing into said upstream side catalyst continuously or intermittently to leaner than a stoichiometric air-fuel ratio, when an air-fuel ratio detected by said downstream side air-fuel ratio detecting means is equal to or less than a rich judged air-fuel ratio, which is richer than the stoichiometric air-fuel ratio, until said oxygen storage amount of the upstream side catalyst becomes a given upstream side judged reference storage amount smaller than a maximum oxygen storage amount; a normal period rich control means for setting said target air-fuel ratio continuously or intermittently to richer than a stoichiometric air-fuel ratio, when said oxygen storage amount of the upstream side catalyst is equal to or greater than said upstream side judged reference storage amount so that said oxygen storage amount decreases toward zero without reaching the maximum oxygen storage amount; and a storage amount recovery control means for setting said target air-fuel ratio continuously or intermittently to leaner than the stoichiometric air-fuel ratio, when the oxygen storage amount of said downstream side catalyst which was estimated by said storage amount estimating means is equal to or less than a given downstream side lower limit storage amount, which is smaller than the maximum storage amount, so that the air-fuel ratio of the exhaust gas flowing out from said upstream side catalyst never becomes richer than the stoichiometric air-fuel ratio but continuously or intermittently becomes leaner than the stoichiometric air-fuel ratio, without setting the target air-fuel ratio by said normal period rich control means and normal period lean control means. a downstream side air-fuel ratio detecting means which is provide in said exhaust passage (19, 22) between said upstream side catalyst and said downstream side catalyst; and an inflow air-fuel ratio control device which controls an air-fuel ratio of exhaust gas flowing into said upstream side catalyst so that said air-fuel ratio of the exhaust gas becomes a target air-fuel ratio; said control system characterized by comprising a storage amount estimating means for estimating an oxygen storage amount of said downstream side catalyst; a normal period lean control means for setting said target air-fuel ratio of exhaust gas flowing into said upstream side catalyst continuously or intermittently to leaner than a stoichiometric air-fuel ratio, when an air-fuel ratio detected by said downstream side air-fuel ratio detecting means is equal to or less than a rich judged air-fuel ratio, which is richer than the stoichiometric air-fuel ratio, until said oxygen storage amount of the upstream side catalyst becomes a given upstream side judged reference storage amount smaller than a maximum oxygen storage amount; a normal period rich control means for setting said target air-fuel ratio continuously or intermittently to richer than a stoichiometric air-fuel ratio, when said oxygen storage amount of the upstream side catalyst is equal to or greater than said upstream side judged reference storage amount so that said oxygen storage amount decreases toward zero without reaching the maximum oxygen storage amount; and a storage amount recovery control means for setting said target air-fuel ratio continuously or intermittently to leaner than the stoichiometric air-fuel ratio, when the oxygen storage amount of said downstream side catalyst which was estimated by said storage amount estimating means is equal to or less than a given downstream side lower limit storage amount, which is smaller than the maximum storage amount, so that the air-fuel ratio of the exhaust gas flowing out from said upstream side catalyst never becomes richer than the stoichiometric air-fuel ratio but continuously or intermittently becomes leaner than the stoichiometric air-fuel ratio, without setting the target air-fuel ratio by said normal period rich control means and normal period lean control means.