An estimation device for a cylinder intake air amount in an internal combustion engine is obtained which can estimate an amount of air actually sucked into a cylinder with a high degree of accuracy by using a physical model of an intake system. A volumetric efficiency corresponding value is calculated based on an exhaust efficiency (a linear function of intake pipe internal pressure) which is an index indicating an amount of residual gas which is an exhaust gas after combustion remaining in the cylinder without being discharged from the interior of the cylinder to an exhaust pipe, and an intake efficiency (a linear function of intake pipe internal pressure) which is an index indicating an amount of air coming into the cylinder from an intake pipe excluding the residual gas, whereby the estimation is carried out to a high degree of accuracy with a small number of adaptation constants.