A cylinder intake air quantity determination device is configured to calculate a manifold internal air quantity based on an inflow air quantity and an outflow air quantity of an intake manifold. Then, a manifold internal pressure is calculated based on the manifold internal air quantity and an intake air temperature, and a cylinder internal pressure corresponding to a crank angle is calculated based on the previous cylinder intake air quantity. The cylinder intake air quantity is then calculated based on the manifold internal pressure, the cylinder internal pressure, and the intake air temperature. Thus, the cylinder intake air quantity determination device is configured to calculate the cylinder intake air quantity with a good precision.