PURPOSE: To prevent torque shock due to the switching of a set air-fuel ratio at the time when the engine load is high, and improve fuel consumption and output at the time when the engine load is low, by delaying the switching speed of the set air-fuel ratio when the detected engine load is high.

CONSTITUTION: A set air-fuel ration, which differs for each of plural operating ranges, is set by an air-fuel ratio setting means P1. An actual air-fuel ratio is adjusted to become the set air-fuel ratio by an air-fuel ratio adjusting means P2. In such an air-fuel ratio control device, the engine load is detected by an engine load detecting means P3. As a result, when the detected engine load is high, the switching speed of the set air-fuel ratio is altered to be delayed by a switching speed altering means P4. Thus, the prevention of torque shock due to the switching of the set air-fuel ratio at the time when the engine load is high (when torque diffential is large), and the improvement in fuel consumption and output due to rapid switching of the air-fuel ratio at the time when the engine load is low (when torque differential is small), can be achieved.