To suppress torque shocks during a changeover to a lean air-fuel ratio by optimizing the adaptation of the air-fuel ratio to engine output torque.

A throttle opening criterion TVOLE is selected by interpolation from a throttle-opening criterion table employing engine speed NE as parameter (S50). A determination is then made whether a throttle opening angle TVO, even with a hysteresis as the case may be, is less than the throttle opening criterion TVOLE (S52, S54). If this proposition is acceptable, a throttle opening determination flag FTVO is set to '1' (S53) to change the air-fuel ratio on a stoichiometric or rich level to a lean air-fuel ratio in the range wherein the generated engine torque is held substantially constant, with the result that no torque shocks are produced.