A fuel injection characteristic sensing device used in a fuel injection system having a pressure accumulator that accumulates fuel to be supplied to a predetermined injector under pressure, at least one fuel pressure sensor for sensing pressure of the fuel and a fuel passage extending from the pressure accumulator to a fuel injection hole of the injector, comprising: a fuel pressure sensing means for sequentially sensing the fuel pressure at a predetermined point corresponding to the fuel pressure sensor based on at least one output of the at least one fuel pressure sensor; an injection timing detecting means (S31-S33, S41-S43, S51-S53) for detecting a predetermined timing in a series of operations concerning fuel injection of the injector based on the fuel pressure sequentially sensed by the fuel pressure sensing means; and a differentiating means (S22, S23) for applying first or higher order differential processing to fuel pressure data sequentially sensed by the fuel pressure sensing means to obtain differential data including at least one of a differential value at a predetermined timing, a timing indicating a predetermined differential value and a timing at which a differential value indicates a predetermined changing mode, wherein the injection timing detecting means (S31-S33, S41-S43, S51-S53) detects a predetermined timing in a series of operations concerning the fuel injection of the injector based on the differential data obtained by the differentiating means (S22, S23); wherein the differentiating means (S22, S23) obtains a cross point as a timing at which an n-th order differential value of the fuel pressure shifts from a smaller side to a larger side than a predetermined threshold value or shifts from the larger side to the smaller side than the predetermined threshold value based on the n-th order differential value of the fuel pressure in a predetermined period, n being an integer equal to or greater than one, and wherein the injection timing detecting means (S31-S33, S41-S43, S51-S53) detects a predetermined timing in a series of operations concerning the fuel injection of the injector based on the cross point obtained by the differentiating means (S22, S23); characterized in that the at least one fuel pressure sensor senses the pressure of the fuel flowing through an inside of the fuel passage at a predetermined point downstream of a neighborhood of a fuel discharge hole of the pressure accumulator with respect to a fuel flow direction, wherein the injection timing detecting means (S31-S33, S41-S43, S51-S53) detects all of an injection start timing, a maximum injection rate reaching timing at which an injection rate is maximized, a timing at which the injection rate starts decreasing after reaching the maximum injection rate, and an injection end timing of the injector based on the differential data obtained by the differentiating means (S22, S23). a fuel pressure sensing means for sequentially sensing the fuel pressure at a predetermined point corresponding to the fuel pressure sensor based on at least one output of the at least one fuel pressure sensor; an injection timing detecting means (S31-S33, S41-S43, S51-S53) for detecting a predetermined timing in a series of operations concerning fuel injection of the injector based on the fuel pressure sequentially sensed by the fuel pressure sensing means; and a differentiating means (S22, S23) for applying first or higher order differential processing to fuel pressure data sequentially sensed by the fuel pressure sensing means to obtain differential data including at least one of a differential value at a predetermined timing, a timing indicating a predetermined differential value and a timing at which a differential value indicates a predetermined changing mode, wherein the injection timing detecting means (S31-S33, S41-S43, S51-S53) detects a predetermined timing in a series of operations concerning the fuel injection of the injector based on the differential data obtained by the differentiating means (S22, S23); wherein the differentiating means (S22, S23) obtains a cross point as a timing at which an n-th order differential value of the fuel pressure shifts from a smaller side to a larger side than a predetermined threshold value or shifts from the larger side to the smaller side than the predetermined threshold value based on the n-th order differential value of the fuel pressure in a predetermined period, n being an integer equal to or greater than one, and wherein the injection timing detecting means (S31-S33, S41-S43, S51-S53) detects a predetermined timing in a series of operations concerning the fuel injection of the injector based on the cross point obtained by the differentiating means (S22, S23); characterized in that the at least one fuel pressure sensor senses the pressure of the fuel flowing through an inside of the fuel passage at a predetermined point downstream of a neighborhood of a fuel discharge hole of the pressure accumulator with respect to a fuel flow direction, wherein the injection timing detecting means (S31-S33, S41-S43, S51-S53) detects all of an injection start timing, a maximum injection rate reaching timing at which an injection rate is maximized, a timing at which the injection rate starts decreasing after reaching the maximum injection rate, and an injection end timing of the injector based on the differential data obtained by the differentiating means (S22, S23).