PURPOSE: To improve the responsiveness in an unsteady state of an engine by a method wherein an injection pulse width is calculated based on a signal from detecting means of the unsteady state of the engine, thereby controlling the operation of an injection valve.

CONSTITUTION: In a central control and operation unit 8 which receives signals from an intake air amount sensor 4 provided upstream a throttle valve 5 of the engine 1, a pressure sensor 7 at downstream thereof and an engine speed sensor 12, and outputs a signal to control a fuel injection valve 6 through a drive circuit 9, it is discriminated whether the engine 1 is driven steadily or not based on the information from the sensor 4 and the like, and if the engine is in an unsteady state, an injection pulse width is calculated and a timer is set to generate an interruption after a certain period of time. When the interruption enters in response to the signal from the engine speed sensor 12, the fuel injection valve is opened for predetermined period of time. Further, in case of the interruption of the timer being set, the valve 6 is opened at a certain pulse width or the pulse width corresponding to the input from the sensor 4 and the like. Thus, the fuel injection valve 6 can be opened at a particular time interval when the engine 1 is driven in an unsteady condition.