PURPOSE: To control the fuel injection amount constant regardless of the back- pressure by maintaining the pressure difference of fluids injected by opening and closing valves at a constant value within a specific scope, in the system where two valves are installed in a nozzle body to which two sorts of fluids are led in.

CONSTITUTION: To a nozzle body 4 connected to a nozzle housing 2 furnishing fluid passages 7 and 8 through which two sorts of fluids, a compressed air and a liquid fuel, for example, flow, a compressed air passage 13 and an air reserving chamber 14 are formed, and a gaseous fuel passage 8 and a spring chamber 11 are also provided. And, the first value 18 are housed in a nozzle housing 2, a distance piece 3, and a nozzle body 4, and the second valve 19 is housed in a nozzle body 4, reciprocatable respectively. In this case, a differential pressure variable pressure-regulator (not shown in the figure) is provided to a fuel pressure piping system connected to a fuel inlet connector 10 is provided so as to maintain the pressure difference between the fuel pressure fed to an injection nozzle 1 and the air pressure at a constant value within a specific scope.