A control system controls a spark-ignition internal combustion engine. The spark-ignition internal combustion engine includes a cylinder, and a fuel injection valve arranged in the cylinder; and an intake port that communicates with the cylinder, and a fuel injection valve arranged in the intake port. Fuel is injected from both the fuel injection valves in at least a part of an operating range. If combustion fluctuations exceed a predetermined amount, in consideration of a preset proportion of fuels to be injected into the cylinder and the intake port, a proportion of fuel to be injected into the cylinder is decreased and a proportion of fuel to be injected into the intake port is increased.