A control system for an internal combustion engine having a throttle valve disposed in an intake passage of said engine, and an evaporative fuel passage for supplying an evaporative fuel/air mixture to said intake passage, the evaporative fuel/air mixture being a mixture of air and evaporative fuel generated in a fuel tank for supplying fuel to said engine, said control system comprising: rotational speed detecting means for detecting an rotational speed of said engine; and intake pressure detecting means for detecting an intake pressure of said engine; characterized by further comprising: wide-open intake air amount calculating means for calculating a wide-open intake air amount (GAWOT) according to the engine rotational speed, the wide-open intake air amount (GAWOT) being an intake air amount corresponding to a state where said throttle valve is fully opened; theoretical intake air amount calculating means for calculating a theoretical intake air amount (GATH) according to the wide-open intake air amount (GAWOT) and the intake pressure, the theoretical intake air amount (GATH) being an intake air amount corresponding to a state where no exhaust gas of said engine is recirculated to a combustion chamber of said engine; intake air amount obtaining means for detecting or estimating an intake air amount (GAIRCYL) of said engine; evaporative fuel/air mixture amount calculating means for calculating an amount (GPGC) of the evaporative fuel/air mixture supplied through said evaporative fuel passage to said intake passage; intake gas amount calculating means for calculating an intake gas amount (GINGASCYL) by correcting the intake air amount (GAIRCYL) using the evaporative fuel/air mixture amount (GPGC); and exhaust gas recirculation ratio calculating means for calculating an exhaust gas recirculation ratio (REGRT) using the theoretical intake air amount (GATH) and the intake gas amount (GINGASCYL), wherein said engine is controlled using the exhaust gas recirculation ratio (REGRT). rotational speed detecting means for detecting an rotational speed of said engine; and intake pressure detecting means for detecting an intake pressure of said engine; characterized by further comprising: wide-open intake air amount calculating means for calculating a wide-open intake air amount (GAWOT) according to the engine rotational speed, the wide-open intake air amount (GAWOT) being an intake air amount corresponding to a state where said throttle valve is fully opened; theoretical intake air amount calculating means for calculating a theoretical intake air amount (GATH) according to the wide-open intake air amount (GAWOT) and the intake pressure, the theoretical intake air amount (GATH) being an intake air amount corresponding to a state where no exhaust gas of said engine is recirculated to a combustion chamber of said engine; intake air amount obtaining means for detecting or estimating an intake air amount (GAIRCYL) of said engine; evaporative fuel/air mixture amount calculating means for calculating an amount (GPGC) of the evaporative fuel/air mixture supplied through said evaporative fuel passage to said intake passage; intake gas amount calculating means for calculating an intake gas amount (GINGASCYL) by correcting the intake air amount (GAIRCYL) using the evaporative fuel/air mixture amount (GPGC); and exhaust gas recirculation ratio calculating means for calculating an exhaust gas recirculation ratio (REGRT) using the theoretical intake air amount (GATH) and the intake gas amount (GINGASCYL), wherein said engine is controlled using the exhaust gas recirculation ratio (REGRT). wide-open intake air amount calculating means for calculating a wide-open intake air amount (GAWOT) according to the engine rotational speed, the wide-open intake air amount (GAWOT) being an intake air amount corresponding to a state where said throttle valve is fully opened; theoretical intake air amount calculating means for calculating a theoretical intake air amount (GATH) according to the wide-open intake air amount (GAWOT) and the intake pressure, the theoretical intake air amount (GATH) being an intake air amount corresponding to a state where no exhaust gas of said engine is recirculated to a combustion chamber of said engine; intake air amount obtaining means for detecting or estimating an intake air amount (GAIRCYL) of said engine; evaporative fuel/air mixture amount calculating means for calculating an amount (GPGC) of the evaporative fuel/air mixture supplied through said evaporative fuel passage to said intake passage; intake gas amount calculating means for calculating an intake gas amount (GINGASCYL) by correcting the intake air amount (GAIRCYL) using the evaporative fuel/air mixture amount (GPGC); and exhaust gas recirculation ratio calculating means for calculating an exhaust gas recirculation ratio (REGRT) using the theoretical intake air amount (GATH) and the intake gas amount (GINGASCYL), wherein said engine is controlled using the exhaust gas recirculation ratio (REGRT).