A control system for an internal combustion engine comprising: an EGR device (11, 15) that has an EGR passage connected at its one end to an exhaust system (6, 7) of the internal combustion engine, and at its other end to an intake system (4, 5) of the internal combustion engine, and serves to introduce a part of an exhaust gas flowing through the exhaust system (6, 7) into the intake system (4, 5) through the EGR passage as an EGR gas; an NOx selective reduction catalyst that is arranged in the exhaust system (6, 7); an ammonia derived compound addition means that serves to add an ammonia derived compound, which is a reducing agent, into the exhaust gas, and is arranged in the exhaust system (6, 7) at the upstream side of said NOx selective reduction catalyst and in such a position as to enable at least a part of the ammonia derived compound thus added to arrive at a connection portion of said EGR passage; and suppression means that suppresses the inflow of the ammonia derived compound added from said ammonia derived compound addition means into said EGR passage; wherein by decreasing a flow rate of the EGR gas flowing through said EGR passage at the time when the ammonia derived compound is added from said ammonia derived compound addition means, in comparison with the case when the ammonia derived compound is not added, said suppression means suppresses the inflow of the ammonia derived compound added from said ammonia derived compound addition means into said EGR passage; and wherein said EGR passage is a low pressure EGR passage that has one end thereof connected to a portion of the exhaust system (6, 7) at the downstream side of a turbine of a turbocharger, and the other end thereof connected to a portion of the intake system (4, 5) at the upstream side of a compressor of the turbocharger; characterized in that said EGR device (11, 15) further includes a high pressure EGR passage that has one end thereof connected to a portion of the exhaust system (6, 7) at the upstream side of the turbine of the turbocharger, and the other end thereof connected to a portion of the intake system (4, 5) at the downstream side of the compressor of the turbocharger; and at the time of decreasing the flow rate of the EGR gas flowing through said low pressure EGR passage when the ammonia derived compound is added from said ammonia derived compound addition means, suppression means decreases the flow rate of the EGR gas flowing through said high pressure EGR passage in cases where the circulation of the EGR gas in said low pressure EGR passage is not stopped. an EGR device (11, 15) that has an EGR passage connected at its one end to an exhaust system (6, 7) of the internal combustion engine, and at its other end to an intake system (4, 5) of the internal combustion engine, and serves to introduce a part of an exhaust gas flowing through the exhaust system (6, 7) into the intake system (4, 5) through the EGR passage as an EGR gas; an NOx selective reduction catalyst that is arranged in the exhaust system (6, 7); an ammonia derived compound addition means that serves to add an ammonia derived compound, which is a reducing agent, into the exhaust gas, and is arranged in the exhaust system (6, 7) at the upstream side of said NOx selective reduction catalyst and in such a position as to enable at least a part of the ammonia derived compound thus added to arrive at a connection portion of said EGR passage; and