1. An air-fuel ratio control system for an internal combustion engine, comprising: an engine condition detecting unit detecting an engine operating condition of the internal combustion engine; and a control unit coupled to the engine condition detecting unit, the control unit being arranged, to calculate a target engine torque on the basis of the engine operating condition, to calculate a target EGR ratio, a target excess air ratio and a target intake air quantity on the basis of the engine operating condition and the target engine torque, to calculate a target equivalence ratio on the basis of the target EGR ratio and the target excess air ratio, to calculate a target injection quantity on the basis of the engine operating condition and the target equivalence ratio, to control an air-fuel ratio at a desired value by bringing a real intake air quantity to the target intake air quantity and by bringing a real fuel injection quantity to the target fuel injection quantity.