A fuel injection device of a common rail type fuel injection system for an engine, wherein at least one fuel passage pressure sensor (20a, 20b, 20c, 20d, 20e, 20f) disposed in a fuel passage between a point of a pipe connected to a fuel discharge port of a common rail apart from the common rail by a certain distance and a fuel injection hole of an injector connected to the pipe, the sensor (20a, 20b, 20c, 20d, 20e, 20f) measuring a fuel pressure at a position where the sensor (20a, 20b, 20c, 20d, 20e, 20f) is disposed; and a pressure fluctuation sensing device (S31, S32, S331, S332, S34, S35, S36, S351, S361, S362, S37, S371, S38) for sensing a pressure fluctuation caused by at least one of an injection operation of the injector and actual injection actually performed through the injection operation based on a sensor output from the fuel passage pressure sensor (20a, 20b, 20c, 20d, 20e, 20f) characterized by: a fuel pressure serially obtaining device for serially obtaining fuel pressure signals of an injector connected to a common rail at intervals short enough to draw a track of a pressure transition waveform of the pressure, an injection characteristic estimating device (S351, S361, S362, S371) for estimating injection characteristics of the injector based on the pressure fluctuation sensed by the pressure fluctuation sensing device (S31, S32, S331, S332, S34, S35, S36, S351, S361, S362, S37, S371, S38), wherein the injection characteristic estimated by the injection characteristic estimating device (S351, S361, S362, S371) is a fluctuation of the injection rate in an injection period of the injector. at least one fuel passage pressure sensor (20a, 20b, 20c, 20d, 20e, 20f) disposed in a fuel passage between a point of a pipe connected to a fuel discharge port of a common rail apart from the common rail by a certain distance and a fuel injection hole of an injector connected to the pipe, the sensor (20a, 20b, 20c, 20d, 20e, 20f) measuring a fuel pressure at a position where the sensor (20a, 20b, 20c, 20d, 20e, 20f) is disposed; and a pressure fluctuation sensing device (S31, S32, S331, S332, S34, S35, S36, S351, S361, S362, S37, S371, S38) for sensing a pressure fluctuation caused by at least one of an injection operation of the injector and actual injection actually performed through the injection operation based on a sensor output from the fuel passage pressure sensor (20a, 20b, 20c, 20d, 20e, 20f) characterized by: a fuel pressure serially obtaining device for serially obtaining fuel pressure signals of an injector connected to a common rail at intervals short enough to draw a track of a pressure transition waveform of the pressure, an injection characteristic estimating device (S351, S361, S362, S371) for estimating injection characteristics of the injector based on the pressure fluctuation sensed by the pressure fluctuation sensing device (S31, S32, S331, S332, S34, S35, S36, S351, S361, S362, S37, S371, S38), wherein the injection characteristic estimated by the injection characteristic estimating device (S351, S361, S362, S371) is a fluctuation of the injection rate in an injection period of the injector. a fuel pressure serially obtaining device for serially obtaining fuel pressure signals of an injector connected to a common rail at intervals short enough to draw a track of a pressure transition waveform of the pressure, an injection characteristic estimating device (S351, S361, S362, S371) for estimating injection characteristics of the injector based on the pressure fluctuation sensed by the pressure fluctuation sensing device (S31, S32, S331, S332, S34, S35, S36, S351, S361, S362, S37, S371, S38), wherein the injection characteristic estimated by the injection characteristic estimating device (S351, S361, S362, S371) is a fluctuation of the injection rate in an injection period of the injector.