An air-fuel ratio control system includes: a catalyst; an A/F sensor provided upstream of the catalyst; an oxygen concentration sensor provided downstream of the catalyst; an output value estimation portion that estimates the output value of the oxygen concentration sensor using a model related to the catalyst and the oxygen concentration sensor; an integral value calculation portion that calculates an integral value of deviation being updated by integrating the difference between the actual oxygen concentration output value and the estimated output value; a correction value calculation portion that calculates a feedback correction value for the output value of the A/F sensor and a target air-fuel ratio at least based on the integral value of deviation; and an air-fuel ratio control portion that zeros a first deviation that is obtained by correcting the difference between the detected air-fuel ratio and the target air-fuel ratio, using the feedback correction value.