Certain intervals are determined in which output signals from an exhaust gas sensing device are sensed, and then applied to a control circuit, to control the fuel supply system (carburetor, or fuel injection system) in such a direction that a predetermined ratio of fuel and air is being supplied to the engine to provide exhaust gases of predetermined composition. The intervals are determined by a logic circuit which has applied thereto engine operating parameters such as operating time of the engine after having been started, engine speed, engine power condition (idling, or supplying power), engine temperature, or the like.