PURPOSE: To prevent lubricating oil from infiltrating into a fuel injection device so as to well maintain injection performance by interposing a gas-liquid separator in a compressed air passage reaching the fuel injection device from an air pump, in the case of an engine provided with the fuel injection device for injecting fuel with compressed air into a combustion chamber.

CONSTITUTION: An electromagnetically opening/closing fuel injection device 46 is mounted to a cylinder head 30 to respectively feed pressure fuel by a fuel pump 50 and compressed air by an air pump 51. The air pump 50 is driven by associated with an engine 16 to suck the outside air through an air cleaner 36 and an air pipe 60 and to supply the compressed air to the fuel injection device 46 through the first/second compressed air pipes 61, 62. Here, a gas/liquid separator 65 is interposed between the first/second compressed air pipes 61, 62. In a process that compressed air, flowing in an up stream chamber 68, once collides against a baffle plate 67 to change the flow downward and to flow into a downstream chamber 69 inversely upward while detouring around a lower end of the baffle plate 67, lubricating oil is separated from the compressed air by a difference between inertia forces.