PURPOSE: To sufficiently purify exhaust gas, by providing means for judging the activation of an O2 sensor and means for increasing the idle rotational speed of the engine so that the idle rotational speed is increased in a predetermined range.

CONSTITUTION: An engine control device has a supply air-fuel ratio control function by the feed-back control of the output air-fuel ratio. A control unit 11 slices an output signal from an O2 sensor 7 at predetermined levels to judge the activation of the O2 sensor 7. When the engine is in its idle condition and its rotational speed is below a predetermined value, the value of a timer is checked. If the value of the timer is below a predetermined value and the timer is clear, the supply of electrical current to an idle-up solenoid 14 is initiated. As a result the O2 feed-back is stably effected so that it makes possible to aim at sufficiently purifying exhaust gas.