Abstract: An air-fuel ratio control system for an internal combustion engine includes an air-fuel ratio sensor arranged in the exhaust system, for detecting the air-fuel ratio of exhaust gases from the engine. An ECU calculates an air-fuel ratio control amount, based on the output from the air-fuel ratio sensor, by using an adaptive controller having an adaptive parameter-adjusting mechanism for adjusting adaptive parameters, such that the air-fuel ratio of a mixture supplied to the engine is converged to a desired air-fuel ratio, and controls the air-fuel ratio of the mixture in a feedback manner, according to the calculated air-fuel ratio control amount. The adaptive parameter-adjusting mechanism calculates a parameter corresponding to an integral term of the air-fuel ratio control amount, and limit-checks a value of the parameter corresponding to the integral term so as to fall within a predetermined limit range.