An electronically controlled fuel injection system including an air flow sensor disposed in the intake pipe of an internal combustion engine upstream of its throttle valve to detect the amount of air flow to the engine and a control unit responsive to the output signal of the air flow sensor to adjust the amount of fuel to be supplied to the engine, is further provided with a fuel enrichment circuit responsive to the output signal of the air flow sensor whereby when the amount of air flow changes in a direction to decrease, a signal commanding an increase in fuel amount which is proportional to the rate of the change is fed to the control unit. The fuel enrichment circuit has an upper limit value such that the increase in fuel amount is prevented from exceeding an amount corresponding to the upper limit value. The fuel enrichment circuit is adapted so that the amount of fuel is not increased when the rate of change of the air flow in the decreasing direction is smaller than a predetermined lower limit value.