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 characterized by comprising: a downstream side air-fuel ratio detection device which is arranged at a downstream side, in the direction of flow of exhaust, of said exhaust purification catalyst and which detects the air-fuel ratio of the exhaust gas which flows out from said exhaust purification catalyst, and an air-fuel ratio control system which controls said air-fuel ratio of the exhaust gas so that said air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes a target air-fuel ratio, the air-fuel ratio control system comprising: an air-fuel ratio lean switching means changing said target air-fuel ratio to a lean set air-fuel ratio which is leaner than a stoichiometric air-fuel ratio, when an exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes a rich air-fuel ratio; a lean degree lowering means changing said target air-fuel ratio to a lean air-fuel ratio with a smaller difference from the stoichiometric air-fuel ratio than said lean set air-fuel ratio, at a timing after said air-fuel ratio lean switching means changes the air-fuel ratio and before the exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes the lean air-fuel ratio; an air-fuel ratio rich switching means changing said target air-fuel ratio to a rich set air-fuel ratio which is richer than the stoichiometric air-fuel ratio, when the exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes the lean air-fuel ratio; and a rich degree lowering means changing said target air-fuel ratio to a rich air-fuel ratio with a smaller difference from the stoichiometric air-fuel ratio than said rich set air-fuel ratio, at a timing after said air-fuel ratio lean switching means changes the air-fuel ratio and before the exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes the rich air-fuel ratio. the control system characterized by comprising: a downstream side air-fuel ratio detection device which is arranged at a downstream side, in the direction of flow of exhaust, of said exhaust purification catalyst and which detects the air-fuel ratio of the exhaust gas which flows out from said exhaust purification catalyst, and an air-fuel ratio control system which controls said air-fuel ratio of the exhaust gas so that said air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes a target air-fuel ratio, the air-fuel ratio control system comprising: an air-fuel ratio lean switching means changing said target air-fuel ratio to a lean set air-fuel ratio which is leaner than a stoichiometric air-fuel ratio, when an exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes a rich air-fuel ratio; a lean degree lowering means changing said target air-fuel ratio to a lean air-fuel ratio with a smaller difference from the stoichiometric air-fuel ratio than said lean set air-fuel ratio, at a timing after said air-fuel ratio lean switching means changes the air-fuel ratio and before the exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes the lean air-fuel ratio; an air-fuel ratio rich switching means changing said target air-fuel ratio to a rich set air-fuel ratio which is richer than the stoichiometric air-fuel ratio, when the exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes the lean air-fuel ratio; and a rich degree lowering means changing said target air-fuel ratio to a rich air-fuel ratio with a smaller difference from the stoichiometric air-fuel ratio than said rich set air-fuel ratio, at a timing after said air-fuel ratio lean switching means changes the air-fuel ratio and before the exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes the rich air-fuel ratio. an air-fuel ratio lean switching means changing said target air-fuel ratio to a lean set air-fuel ratio which is leaner than a stoichiometric air-fuel ratio, when an exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes a rich air-fuel ratio; a lean degree lowering means changing said target air-fuel ratio to a lean air-fuel ratio with a smaller difference from the stoichiometric air-fuel ratio than said lean set air-fuel ratio, at a timing after said air-fuel ratio lean switching means changes the air-fuel ratio and before the exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes the lean air-fuel ratio; an air-fuel ratio rich switching means changing said target air-fuel ratio to a rich set air-fuel ratio which is richer than the stoichiometric air-fuel ratio, when the exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes the lean air-fuel ratio; and a rich degree lowering means changing said target air-fuel ratio to a rich air-fuel ratio with a smaller difference from the stoichiometric air-fuel ratio than said rich set air-fuel ratio, at a timing after said air-fuel ratio lean switching means changes the air-fuel ratio and before the exhaust air-fuel ratio detected by said downstream side air-fuel ratio detection device becomes the rich air-fuel ratio.