PURPOSE: To improve the extent of fuel control accuracy and engine performance by detecting the actual fuel flow characteristic of a fuel injection valve on the basis of turnover frequency of an air-fuel ratio feedback compensation factor within the specified setting period, and making a controller perform the fuel control conformed to it.

CONSTITUTION: At time of engine driving, a fundamental fuel injection quantity is determined on the basis of output value of an air flow meter 2 and engine speed or the like while according to a deviation between air-fuel ratio and desired air-fuel ratio by an oxygen sensor S1, the fundamental fuel injection quantity is feedback-controlled by an engine control unit 9, through which the air-fuel ratio is always maintained to the setting ratio. In this case, the current engine driving state is in a driving state to do feedback control over a fuel injection quantity, and the current counter value of a judging period setting timer counter for judging feedback control value turnover frequency or amplitude of the fuel injection quantity is read at time of judging an idle range. A fuel flow characteristic in a fuel injection valve 5 is detected on the basis of the turnover frequency within the specified period of the feedback control value, making it conductive to fuel control.