PURPOSE: To prevent the temperature rise of fuel inside a tank by circulating the fuel pressurized by a feed pump inside the gallery of a cylinder head, and by cooling the surplus fuel being returned from the gallery to the inside of the tank by means of a cooling means.

CONSTITUTION: When a feed pump 24 is driven by an engine via a gear 23, the fuel sucked from a tank 26 is pressurized to a low pressure and is introduced into the gallery 31 of a cylinder head 13, and the fuel inside the gallery 31 is filled into the injectors 14 of the cylinder head 13. When the cams 20 on a cam-shaft 19 push the plungers 18 of the unit injectors 14, fuel is injected towards the combustion chambers 15 of respective pistons 12 through the spray nozzles at the ends of respective injectors. The surplus fuel is returned to the tank 26 through a return tube 32. At the time, fuel is returned after it has been cooled by a fuel cooling cooler 33. Thus, the temperature of fuel inside the tank 26 can be prevented from becoming a high temperature.