An object of the invention is to provide a fuel injection technique suitable for a spark-ignition internal combustion engine equipped with a first fuel injection valve for injecting fuel into a cylinder, a second fuel injection valve for injecting fuel into an intake passage, and a particulate filter provided in an exhaust passage thereof. To achieve the object, the fuel injection system of an internal combustion engine according to the invention reduces an in-cylinder injection ratio, which is the ratio of the quantity of fuel injected through the first fuel injection valve to the quantity of fuel injected through the second fuel injection valve, when the quantity of particulate matter trapped in the particulate filter is larger than a threshold value, thereby reducing the quantity of particulate matter discharged from the internal combustion engine.