Abstract: An air-fuel ratio control system for an engine, to which evaporated fuel in a canister is supplied, is used for feedback controlling an air-fuel ratio based on a feedback control value calculated according to the quantity of evaporated fuel supplied to the engine. A calculation is made, in a first range of engine operating conditions, of a difference of a first control value between the first control value determined during supplying of the evaporated fuel to the engine and the first control value determined during not supplying of the evaporated fuel to the engine in the first range. A second control value for a second range of engine operating conditions is established different from the first range based on a feedback control value for controlling an air-fuel ratio of fuel in the first range, a ratio of the quantity of intake air into the engine between the first and second ranges, and a ratio of the supplying quantity of the evaporated fuel into the engine between the first and second ranges.