Cylinder intake air quantity calculation device

A cylinder intake air quantity determination device is configured to calculate a manifold internal air quantity (Cm) based on an inflow air quantity (Qa) and an outflow air quantity (Cc(n-1)) of an intake manifold (8). Then, a manifold internal pressure (Pm) is calculated based on the manifold internal air quantity (Cm) and an intake air temperature (Tm), and a cylinder internal pressure (Pc) corresponding to a crank angle (Ca) is calculated based on the previous cylinder intake air quantity (Cc(n-1)). The cylinder intake air quantity (Cc) is then calculated based on the manifold internal pressure (Pm), the cylinder internal pressure (Pc), and the intake air temperature (Tm). Thus, the cylinder intake air quantity determination device is configured to calculate the cylinder intake air quantity (Cc) with a good precision.