A fuel injection system comprising: a fuel supply pump for pressurizing a fuel to constitute a high pressure; an injector for supplying to inject the fuel at the high pressure delivered from the fuel supply pump to a respective cylinder of an engine; and injection amount controlling means for calculating a command injection amount and an injection timing in accordance with an engine operating condition and for driving the injector in accordance with the calculated command injection amount and the calculated injection timing, wherein the fuel injection system is capable of carrying out a multi-injection for injecting the fuel in one cycle of the engine in a plurality of times, and the injection amount controlling means being characterized by injection time period determining means for calculating a basic injection time period of a respective fuel injection of the multi-injection from a map or an equation showing a relationship between a fuel injection amount and an injection time period set by assuming fuel injection at a predetermined angle at a vicinity of a top dead center of the engine; injection start angle calculating means for calculating a respective injection start angle of the multi-injection from the injection timing and the basic injection time period; combustion chamber pressure calculating means (S24, S25) for calculating a combustion chamber pressure when the respective fuel injection of the multi-injection is started by a map or an equation showing a relationship between the injection start angle and the combustion chamber pressure; and correcting means (S27, S28) for correcting the basic injection time period of the respective fuel injection of the multi-injection in accordance with an amount of a change between the combustion chamber pressure calculated based on the injection start angle and the assumed combustion chamber pressure assumed in calculating the basic injection time period. a fuel supply pump for pressurizing a fuel to constitute a high pressure; an injector for supplying to inject the fuel at the high pressure delivered from the fuel supply pump to a respective cylinder of an engine; and injection amount controlling means for calculating a command injection amount and an injection timing in accordance with an engine operating condition and for driving the injector in accordance with the calculated command injection amount and the calculated injection timing, wherein the fuel injection system is capable of carrying out a multi-injection for injecting the fuel in one cycle of the engine in a plurality of times, and the injection amount controlling means being characterized by injection time period determining means for calculating a basic injection time period of a respective fuel injection of the multi-injection from a map or an equation showing a relationship between a fuel injection amount and an injection time period set by assuming fuel injection at a predetermined angle at a vicinity of a top dead center of the engine; injection start angle calculating means for calculating a respective injection start angle of the multi-injection from the injection timing and the basic injection time period; combustion chamber pressure calculating means (S24, S25) for calculating a combustion chamber pressure when the respective fuel injection of the multi-injection is started by a map or an equation showing a relationship between the injection start angle and the combustion chamber pressure; and correcting means (S27, S28) for correcting the basic injection time period of the respective fuel injection of the multi-injection in accordance with an amount of a change between the combustion chamber pressure calculated based on the injection start angle and the assumed combustion chamber pressure assumed in calculating the basic injection time period.