A fuel supply system for an internal combustion engine is disclosed which includes a single fuel injection valve provided at the area at which a plurality of branch passages of an intake manifold join, and a pressure regulator which regulates the pressure of the fuel supplied to the fuel injection valve in accordance with a vacuum pressure in an intake manifold.According to this system, during low load condition, the pressure of the fuel supplied to the fuel injection valve is held within a range of levels lower than that appearing during high load conditions, thereby increasing the duration during which the fuel injection valve is opened in synchronism with the operation of each cylinder. Thereby, this system makes it possible to uniformly supply fuel to each cylinder for all running conditions of the vehicle.