A control system of an internal combustion engine provided with: a throttle valve passage air flow calculation equation by which a throttle valve passage air flow (mt) is expressed as a function of a downstream side intake pipe pressure (Pm) at the downstream side of a throttle valve, and a cylinder intake air flow calculation equation by which a cylinder intake air flow (mc) is expressed as a function of said downstream side intake pipe pressure (Pm), wherein said internal combustion engine has an exhaust gas recirculation passage for making at least part of the exhaust gas discharged into the exhaust passage flow into the intake passage and an EGR control valve for adjusting the flow of the exhaust gas passing through said exhaust gas recirculation passage, said control system being characterized in that the control system is configured to calculate said downstream side intake pipe pressure (Pm) when the throttle valve passage air flow (mt) found from said throttle valve passage air flow calculation equation and the cylinder intake air flow (mc) found from said cylinder intake air flow calculation equation match as the downstream side intake pipe pressure (Pmta) at the time of steady operation under the operating conditions at that time, and to control the internal combustion engine based on the calculated downstream side intake pipe pressure (Pmta), and said throttle valve passage air flow calculation equation is expressed as the following equation wherein mt is a throttle valve passage air flow, μ is a flow coefficient at the throttle valve, At is a cross-sectional area of the opening of the throttle valve, Pa is an atmospheric pressure, Ta is an atmospheric temperature, R is a gas constant, Pm is said downstream side intake pipe pressure, and Φ(Pm/Pa) is a coefficient determined in accordance with the value of Pm/Pa, and said cylinder intake air flow calculation equation is expressed as the following equation where mc is a cylinder intake air flow, and e and g are compliance parameters determined based on at least the engine speed and the opening degree of said EGR control valve: mt = μ ⋅ At ⋅ Pa R ⋅ Ta ⋅ Φ Pm Pa mc = e ⋅ Pm + g a throttle valve passage air flow calculation equation by which a throttle valve passage air flow (mt) is expressed as a function of a downstream side intake pipe pressure (Pm) at the downstream side of a throttle valve, and a cylinder intake air flow calculation equation by which a cylinder intake air flow (mc) is expressed as a function of said downstream side intake pipe pressure (Pm), wherein said internal combustion engine has an exhaust gas recirculation passage for making at least part of the exhaust gas discharged into the exhaust passage flow into the intake passage and an EGR control valve for adjusting the flow of the exhaust gas passing through said exhaust gas recirculation passage, said control system being characterized in that the control system is configured to calculate said downstream side intake pipe pressure (Pm) when the throttle valve passage air flow (mt) found from said throttle valve passage air flow calculation equation and the cylinder intake air flow (mc) found from said cylinder intake air flow calculation equation match as the downstream side intake pipe pressure (Pmta) at the time of steady operation under the operating conditions at that time, and to control the internal combustion engine based on the calculated downstream side intake pipe pressure (Pmta), and said throttle valve passage air flow calculation equation is expressed as the following equation wherein mt is a throttle valve passage air flow, μ is a flow coefficient at the throttle valve, At is a cross-sectional area of the opening of the throttle valve, Pa is an atmospheric pressure, Ta is an atmospheric temperature, R is a gas constant, Pm is said downstream side intake pipe pressure, and Φ(Pm/Pa) is a coefficient determined in accordance with the value of Pm/Pa, and said cylinder intake air flow calculation equation is expressed as the following equation where mc is a cylinder intake air flow, and e and g are compliance parameters determined based on at least the engine speed and the opening degree of said EGR control valve: mt = μ ⋅ At ⋅ Pa R ⋅ Ta ⋅ Φ Pm Pa mc = e ⋅ Pm + g