PURPOSE: To make it possible to control the pressure of fuel at a low value to enlarge the control range of fuel pressure, and to enable an appropriate fuel supply in accordance with various operating conditions, by providing such as arrangement that vacuum is effected in a return passage for fuel discharged from a fuel chamber in a fuel pressure regulator.

CONSTITUTION: A fuel pressure regulator 28 is composed of a fuel chamber 31 and an air chamber 32 in a fuel passage 27, a diaphragm partitioning separating between the above-mentioned chambers 31, 32 and a relief valve 34 fixed to the center of the diaphragm 33, for steplessly changing the opening area of a return passage 29. Further, the fuel pressure regulator 28 is adapted to feed fuel into a fuel injection valve 24 disposed in an intake-air passage 22. In the above-mentioned arrangement, a pump 35, as a vacuum generating means 30, is disposed in the return passage 29. Further, when the pump 35 is driven, vacuum is effected in the return passage 29 on the fuel pressure regulator 28 side so that is is possible to control the fuel pressure in the fuel chamber 31 at a low value around the atmospheric pressure.