@ A fuel injection system for an internal combustion engine has a fuel injector (10) disposed in a main air passage (33) upstream of a throttle valve (18). A part of air flowing through the main air passage is caused to flow through a by-pass air passage (5, 14) having outlet ports (8) open to the main air passage upstream of the injection orifice of the injector (10). A thermal type air flow meter (7, 9) measures the rate of air flow through the by-pass air passage. In order to prevent whirling-up of injected fuel and resultant adhesion of the fuel to walls adjacent to the by-pass air passage outlet ports (8), the cross-sectional area of the main air passage adjacent to the by-pass air passage outlet ports (8) is determined to be greater than that of the main air passage around the throttle valve (18) when in its fully open position.