In a closed loop control system for air/fuel ratio control of an internal combustion engine, an integration correction factor is derived from the output signal of a gas sensor indicative of the concentration of an exhaust gas component, and an engine condition correction factor is selected from a memory in which a plurality of engine condition correction factors are prestored in the form of a table. The engine condition correction factor is renewed in accordance with the variation in the value of the integration correction factor so as to perform learning control. In order to prevent each of the engine condition correction factors from assuming an undesirable value which are far deviated from its standard, a constant or variable limit value is set. The limit value may be set by calculating some typical engine condition correction factors. The correction factors will be used to modify a basic quantity of fuel to be injected into each cylinder of the engine for feedback controlling the air/fuel ratio of the air/fuel mixture.