PURPOSE: To extend the controllable operation range of an engine, by feedback controlling the air-fuel ratio on the basis of the output signal of an air-fuel ratio sensor of an air-fuel ratio controlling apparatus in the manner than it becomes equal to the theoretical air-fuel ratio when the temperature of the air-fuel ratio sensor is lower than a prescribed value while it is controlled to a desired value other than the theoretical air-fuel ratio when the temperature of said sensor is higher than the prescribed value.

CONSTITUTION: At the time of low-load operation of an engine 1 or just after starting of the engine 1, it is detected from the output signal of a detector 10 that the temperature of exhaust gas passed through an exhaust pipe 7, i.e., the temperature of an air-fuel ratio sensor 8 is low. In this state, an electronic control unit 12 controls the quantity of fuel supplied from a fuel supply valve 5 such that the output signal corresponding to pump current IP of an electronic air-fuel ratio detecting means 9 becomes minimum. Therefore, the air-fuel ratio is feedback controlled to be always equal to the theoretical air-fuel ratio. On the other hand, when the temperature of the air-fuel ratio sensor 8 becomes higher than a prescribed value at the time of high-speed, high-load operation of the engine, it becomes that a desired air-fuel ratio other than the theoretical air-fuel ratio can be detected from the pump current IP. Thus, since the feedback-controlled, aimed air-fuel ratio is varied from the theoretical air-fuel ratio to a desired air-fuel ratio by the electronic control unit 12, it is enabled to extend the controllable operation range of an engine.