To properly prevent the backflow of compressed air into a fuel injection valve in a fuel supply system even if the pressure of the compressed air does not rapidly lower after the stop of an associated internal combustion engine.

A fuel supply system for an internal combustion engine comprises an air blast injector A, of which fuel injection valve involves a switch valve adapted to be held closed against the pressure of fuel regulated by a fuel-pressure regulating means or valve 23 and the pressure of compressed air regulated by an air-pressure regulating means or valve 19 by means of the force of a spring. The fuel supply system is provided with a pressure detection means or pressure switch 51 for detecting the pressure of compressed air discharged from an air pump 11 adapted to be driven or stopped in synchronism with an associated internal combustion engine, and at a fuel pump drive means or device 13 with a holding means for holding a fuel pump 15 driven until the pressure of compressed air detected by the pressure detection means 51 lowers to a set pressure of or below a pressure corresponding to the spring force.