PURPOSE: To conform an air-fuel ratio with a target air-fuel ratio by estimating the new-air partial pressure and exhaust-gas partial pressure of a mixture based on a recirculating exhaust gas quantity estimated from the behavior of an EGR value, the engine speed of an internal combustion engine, a pressure inside an intake air passage, and a new air temperature.

CONSTITUTION: A first estimating means M8 estimates an exhaust gas quantity recirculated into an intake air passage M2 from the behavior of the EGR value of an EGR device M3 which is estimated by a behavior estimating means M7. Then, a second estimating means M9 estimates the new-air partial pressure and an exhaust-gas partial pressure of a mixture based on the estimated recirculating exhaust gas quantity, the engine speed of an internal combustion engine M5 detected by an operating condition detecting means M6, a pressure inside the intake air passage and an intake air temperature. From these, a new air quantity taken into a cylinder at the time of an intake stroke is calculated by a calculating means M10, and a corresponding fuel quantity is calculated by a calculating means M11. Thereby, an air-fuel ratio can be conformed with a target air fuel ratio.