A control system of an internal combustion engine, the engine comprising an exhaust purification catalyst which is arranged in an exhaust passage of the internal combustion engine and which can store oxygen, the control system comprising: a downstream side air-fuel ratio sensor which is arranged at a downstream side of said exhaust purification catalyst in a direction of exhaust flow and which detects an air-fuel ratio of the exhaust gas flowing out from said exhaust purification catalyst; and an air-fuel ratio control device which controls the air-fuel ratio of the exhaust gas so that the air-fuel ratio of the exhaust gas flowing into said exhaust purification catalyst becomes a target air-fuel ratio, wherein said target air-fuel ratio is set to a lean air-fuel ratio which is leaner than a stoichiometric air-fuel ratio when an exhaust air-fuel ratio which is detected by said downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio, which is richer than the stoichiometric air-fuel ratio, or less, and is set to a rich air-fuel ratio which is richer than a stoichiometric air-fuel ratio when an exhaust air-fuel ratio which is detected by said downstream side air-fuel ratio sensor becomes a lean judged air-fuel ratio, which is leaner than the stoichiometric air-fuel ratio, or more; and, when the engine operating state is a steady operation state and is a low load operation state, compared with when the engine operating state is not a steady operation state and is a medium and high load operation state, at least one of an average lean degree of said target air-fuel ratio while said target air-fuel ratio is set to a lean air-fuel ratio, and an average rich degree of said target air-fuel ratio while said target air-fuel ratio is set to a rich air-fuel ratio is increased. a downstream side air-fuel ratio sensor which is arranged at a downstream side of said exhaust purification catalyst in a direction of exhaust flow and which detects an air-fuel ratio of the exhaust gas flowing out from said exhaust purification catalyst; and an air-fuel ratio control device which controls the air-fuel ratio of the exhaust gas so that the air-fuel ratio of the exhaust gas flowing into said exhaust purification catalyst becomes a target air-fuel ratio, wherein said target air-fuel ratio is set to a lean air-fuel ratio which is leaner than a stoichiometric air-fuel ratio when an exhaust air-fuel ratio which is detected by said downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio, which is richer than the stoichiometric air-fuel ratio, or less, and is set to a rich air-fuel ratio which is richer than a stoichiometric air-fuel ratio when an exhaust air-fuel ratio which is detected by said downstream side air-fuel ratio sensor becomes a lean judged air-fuel ratio, which is leaner than the stoichiometric air-fuel ratio, or more; and, when the engine operating state is a steady operation state and is a low load operation state, compared with when the engine operating state is not a steady operation state and is a medium and high load operation state, at least one of an average lean degree of said target air-fuel ratio while said target air-fuel ratio is set to a lean air-fuel ratio, and an average rich degree of said target air-fuel ratio while said target air-fuel ratio is set to a rich air-fuel ratio is increased.