PURPOSE: To prevent high-idling in an engine warm state in a fuel injector for an internal combustion engine of an air assist system by increasing a quantity of assist air.

CONSTITUTION: A fuel injector is provided with a throttle valve 1 fixed to an intake pipeline 15, for controlling an air quantity to be sucked into an internal combustion engine; fuel injection valves 3a-3d exposed at each tip thereof to the internal combustion engine, for injecting required fuel; air introducing passages 4a-4d each connected at one end thereof upstream of the throttle valve 1 while at the other end thereof to the fuel injection valves 3a-3d; solenoid valves 5a-5d fixed to the air introducing passages 4a-4d, for controlling a flow of assist air generated from a source of an intake manifold pressure; and a controller 17 for opening the solenoid valves 5a-5d on the basis of information of a crank angle sensor for detecting a crank angle of the internal combustion engine only when the fuel injection valves 3a-3d perform injection, for supplying the assist air to the internal combustion engine.