Abstract: An apparatus estimates a quantity of recirculated exhaust gas. The apparatus includes a first unit computing a total quantity of gas, a second unit computing a quantity of exhaust gas passing through an EGR vale by means of an EGR valve model, a third unit computing a temporal quantity of the exhaust gas by means of an EGR diffusion model which simulates a behavior of the exhaust gas, a fourth unit obtaining an intake air quantity by subtracting the temporal exhaust gas quantity from the total gas quantity, a fifth unit correcting the computed intake air quantity by means of intake air quantity measured by the airflow meter, a sixth unit computing an intake air pressure based on the corrected quantity of the intake air, and a seventh unit computing a quantity of the recirculated exhaust gas based on at least the intake air pressure.