A fuel injection system comprising: an injector that injects fuel to an internal combustion engine; a controller that controls an injection start timing to start fuel injection from the injector, an injection amount from the injector, and a supply pressure of the fuel supplied to the injector; and a cylinder internal pressure sensor that detects a cylinder internal pressure of the internal combustion engine, wherein the controller includes: real state estimation means for estimating a real heat release rate from the cylinder internal pressure; standard state estimation means for obtaining a standard state, in which a heat release rate of standard fuel having a viscosity and a fuel performance is a target heat release rate, upon fuel injection from the injector; and correction means for correcting at least one of the injection start timing, the injection amount, and the supply pressure in a direction to eliminate a difference that exists between the standard state obtained by the standard state estimation means and the real heat release rate estimated by the real state estimation means, characterized in that the correction means corrects the supply pressure in a direction to eliminate a slope angle difference (Δc) when the slope angle difference (Δc) exists between a rising slope angle of the standard state obtained by the standard state estimation means and a rising slope angle of the real heat release rate estimated by the real state estimation means. an injector that injects fuel to an internal combustion engine; a controller that controls an injection start timing to start fuel injection from the injector, an injection amount from the injector, and a supply pressure of the fuel supplied to the injector; and a cylinder internal pressure sensor that detects a cylinder internal pressure of the internal combustion engine, wherein the controller includes: real state estimation means for estimating a real heat release rate from the cylinder internal pressure; standard state estimation means for obtaining a standard state, in which a heat release rate of standard fuel having a viscosity and a fuel performance is a target heat release rate, upon fuel injection from the injector; and correction means for correcting at least one of the injection start timing, the injection amount, and the supply pressure in a direction to eliminate a difference that exists between the standard state obtained by the standard state estimation means and the real heat release rate estimated by the real state estimation means, characterized in that the correction means corrects the supply pressure in a direction to eliminate a slope angle difference (Δc) when the slope angle difference (Δc) exists between a rising slope angle of the standard state obtained by the standard state estimation means and a rising slope angle of the real heat release rate estimated by the real state estimation means. real state estimation means for estimating a real heat release rate from the cylinder internal pressure; standard state estimation means for obtaining a standard state, in which a heat release rate of standard fuel having a viscosity and a fuel performance is a target heat release rate, upon fuel injection from the injector; and correction means for correcting at least one of the injection start timing, the injection amount, and the supply pressure in a direction to eliminate a difference that exists between the standard state obtained by the standard state estimation means and the real heat release rate estimated by the real state estimation means, characterized in that the correction means corrects the supply pressure in a direction to eliminate a slope angle difference (Δc) when the slope angle difference (Δc) exists between a rising slope angle of the standard state obtained by the standard state estimation means and a rising slope angle of the real heat release rate estimated by the real state estimation means.