An air amount estimating apparatus in an internal combustion engine having a supercharger that estimates with high accuracy an amount of air drawn into a cylinder without involving an increased calculation load. The air amount estimating apparatus estimates a cylinder air amount through calculation that incorporates a physical model representing air flowing behavior through an intake path. When, at this time, a ratio of throttle downstream pressure to throttle upstream pressure as calculated with the physical model is smaller than a reference ratio, a throttle flow rate is calculated using an equation of throttle based on the throttle upstream pressure, the throttle downstream pressure, and a throttle opening. When the ratio of the throttle downstream pressure to the throttle upstream pressure is greater than the reference ratio, the throttle flow rate is calculated using a linear interpolation equation based on a compressor flow rate and an intake valve flow rate.