The fuel injection apparatus of the invention comprises a unit injector for injecting fuel to a combustion chamber of an internal combustion engine. An injection pump chamber and a timing chamber are defined in the unit injector, a quantity of fuel, adjusted in accordance with a drive condition of an engine, being supplied to these chambers. Fuel in the injection pump chamber and the timing chamber can be pressurized by an injection plunger and a timing plunger operated by pressure from the same pressure source. When the fuel in the injection pump chamber is pressurized to a predetermined injection pressure, fuel in the injection pump chamber is injected to the corresponding combustion chamber of the engine. The timing chamber is connected to a relief path, and a check valve, which can be opened at a pressure lower than the injection pressure, is provided in the relief path.