PURPOSE: To sharply reduce separation carbon and a sulfur content by a method wherein a surge tank with a liquid mixing valve to feed light oil mixed in a given ratio with liquefied petroleum gas is mounted to a fuel pressure regulator to feed fuel to a fuel injection pump.

CONSTITUTION: Light oil from a light oil tank is pressurized to the same pressure as a gas pressure of liquefied petroleum gas with the aid of a combustible fuel pressure pump 4 and fed to a surge tank 7 with a liquid mixing valve. Liquefied petroleum gas is fed from a petroleum gas tank 2 to the surge tank 7 with a liquid mixing valve. Light oil and liquefied petroleum gas are mixed together in a desired ratio in the surge tank 7 with a liquid mixing valve. Thereafter, pressurization is effected by means of a fuel pressure regulator 8 so that the mixture fuel is prevented from the occurrence of vapor lock to a fuel injection pump 10a. This system sharply reduces separation carbon and a sulfur content in exhaust gas.