An estimation device for a cylinder intake air amount in an internal combustion engine can estimate a cylinder intake air amount to a sufficient accuracy for suitable engine control, in either of steady state operation and transient operation with a small number of adaptation constants and a small amount of calculation load, without requiring a huge memory capacity. The relation between an opening degree and an effective opening area of a throttle valve has been learned, while calculating the cylinder intake air amount in an S/D method from a volumetric efficiency correction factor map adapted by valve timing at the time of steady state operation, and in a period from a transient change until an exhaust manifold temperature is converged, an intake air amount is calculated based on the relation thus learned. The cylinder intake air amount is calculated based on a physical model of an intake system response delay.