In a feedback control system for air/fuel ratio control of an internal combustion engine, an integration correcting amount is derived from the output signal of a gas sensor indicative of the concentration of an exhaust gas component, and an engine condition correcting amount is selected from a memory in which a plurality of engine condition correcting amounts are prestored in the form of a map. The engine condition correcting amount is renewed in accordance with the variation in the value of the integration correcting amount only within a given period of time or within an interval corresponding to a given number of rotations of the engine crankshaft from an instant of detection of the variation of the gas sensor output level. Thus, the engine condition correcting amount is renewed only when the output signal level of the gas sensor is reliable. In the case that the gas sensor output level does not change for a relatively long period of time, the integration correction amount is set to a standard value, while the engine condition correcting amount is not changed, using prestored data.