PURPOSE: To improve stability at idling by preventing air-fuel ratio from becoming rich or lean due to the effect of fuel sticking quantity on the wall in an intake pipe based on variation of external load.

CONSTITUTION: An engine 1 is provided with an injector 17 for fuel supply, an air flow meter 32, a water temperature sensor 35, and the like. An electronic control unit(ECU) 51 detects starting and then computes a synthetic fuel increase quantity after starting. Further, each load equivalent value is computed based on the intake air quantity and the like, in addition, a no load equivalent value in the operating condition corresponding to the value is computed from a previously memorized map, and a load ratio equivalent to the degree of load is computed from both values. A load fuel increase quantity factor is computed according to the load ratio and reflected on the synthetic fuel increase quantity after starting. Hereby, when a load is added to the engine, fuel is supplied not only corresponding to the load, but corresponding to fine variation of the load equivalent value.