An air-fuel ratio control system for an internal combustion engine, which is capable of accurately estimating an exhaust gas state parameter according to the properties of fuel, thereby making it possible to properly control the air-fuel ratio of a mixture. The air-fuel ratio control system 1 estimates an exhaust gas state parameter indicative of a state of exhaust gases, as an estimated exhaust gas state parameter (AF13 NN) by inputting a detected combustion state parameter (DCADLYIG) indicative of a combustion state of the mixture in the engine 3, and detected operating state parameters (NE, TW, PBA, IGLOG, TOUT) indicative of operating states of the engine 3, to a neural network (NN) configured as a network to which are input the combustion state parameter (DCADLYIG) and the operating state parameters (NE, TW, PBA, IGLOG, TOUT), and in which the exhaust gas state parameter is used as a teacher signal (step 1), and controls the air-fuel ratio based on the estimated exhaust gas state parameter (AF_NN) (steps 3, 4, and 24 to 28).