A control system for an internal combustion engine controls an amount of auxiliary air supplied to the engine in the following manner: Concentration of evaporative fuel contained in a gas purged from an evaporative emission control system into an intake passage of the engine is calculated based on a purging flow rate estimated based on operating conditions of the engine, and an output from a flowmeter arranged in the evaporative emission control system. A volume of the gas purged over a predetermined time period is calculated based on the output from the flowmeter. A parameter indicative of dynamic characteristics of the gas is calculated based on operating conditions of the engine. An amount of air contained in the purged gas drawn into a combustion chamber of the engine is calculated at least based on the concentration of evaporative fuel, the volume of the purged gas, and the parameter indicative of dynamic characteristics of the gas. The amount of auxiliary air supplied to the engine is controlled in a manner dependent on the amount of air contained in the purged gas.