Fuel injection system of internal combustion engine

An engine electronic control unit (engine ECU) (18) obtains a fuel consumption of a vehicle from a fuel consumption measuring device (4) and calculates an assumed fuel consumption of the vehicle if it is determined that an engine (2) enters an exhaustion stroke based on a sensor output of a rotational angle sensor. If the obtained fuel consumption is greater than the assumed fuel consumption, the engine ECU (18) prohibits a post-injection, which does not generate a driving force of the engine (2). Accordingly, a total quantity of the fuel injected from an injector (17) in operation cycles of the engine (2) is reduced. As a result, significant deterioration of the fuel consumption of the vehicle can be prevented. Thus, the fuel injection taking the fuel consumption of the vehicle into consideration can be performed.