An air-fuel ratio control device capable of accurately controlling the air-fuel ratio of the mixture to be supplied to the engine, without being affected by the external conditions such as the atmospheric pressure and the temperature of the air drawn into the engine. The control signal of the air-fuel ratio control device contains a sensitivity component whose magnitude varies in proportion to a time of duration for which the "rich" or "lean" input signal has continued, so that the air-fuel ratio of the mixture can be properly controlled for all engine running conditions, i.e., either when the engine is running under a light load condition or under a heavy load condition.