A fuel injection device 1A which includes a common rail 4 for accumulating fuel delivered by a high pressure pump 3B in a pressure-accumulated state, an injector for injecting in a cylinder of the diesel engine fuel supplied through a high pressure fuel supply passage 21 branched from the common rail 4, and an ECU 80A for outputting an injection command signal for injecting the fuel from the injector 5A. The fuel injection device 1A further includes an orifice 75 in the high pressure fuel supply passage 21 on the side of the common rail 4, and a differential pressure sensor S dP for detecting the pressure difference of the pressures on the upstream and downstream sides of the orifice 75. The ECU 80A calculates an actual fuel supply amount that passes the orifice 75 based on a signal from the differential pressure sensor S dP .