PURPOSE: To improve the fuel efficiency, by using a transient operation judgment means to detect acceleration or deceleration and by using an idling judgment means to confirm that idling is not being performed, to thereafter appropriately compensate the quantity of fuel.

CONSTITUTION: To regulate a fuel injection valve 20 by a controller 50, a basic injected fuel quantity Tp is first calculated from the outputs of an air flow meter 15 and a rotational frequency sensor 29. A fuel increase ratio Te, an acceleration increase ratio Ae and a deceleration decrease ratio Rr, which are determined from the outputs of an intake air temperature sensor 58 and a water temperature sensor 59, are used to calculate a fuel compensation ratio Tc. An injected fuel quantity is calculated on the basis of the basic fuel quantity Tp and the fuel compensation ratio Tc. The acceleration or deceleration of the engine is detected by a transient operation judgment means on the basis of the changes in the flow rate Q of air and the rotational frequency N of the engine. At that time, the acceleration increase ratio Ae or the deceleration decrease ratio Rr is applied to compensate the basic injected fuel quantity Tp.