A fuel injection system for an internal combustion engine comprises a fuel injecting valve, L-jetronic calculating means for calculating the amount of fuel to be injected based upon the output from an air flow meter under an engine operating condition where the amount of intake air is small, D-jetronic calculating means for calculating the amount of fuel to be injected based upon outputs from engine load detecting means and engine rpm detecting means under an engine operating condition where the amount of intake air is large and correcting means for correcting the amount of fuel to be injected upon switchover between the L-jetronic and D-jetronic calculating means in response to engine operating condition changes. As a result, the difference between successive amounts of fuel to be injected is never large. of fuel injected lastly. According to the present invention, it is possible to control the air-flow ratio to the optimum value when the method for calculating the amount of fuel to be injected changes between the L-jetronic method and the D-jetronic method in response to engine operating condition changes, even during an acceleration or deceleration.