A primary passage and a secondary passage in a parallel relation to each other are disposed between an air cleaner and a branch passage collecting portion of an intake manifold. The primary passage is smaller in diameter than the secondary passage and has at the downstream end thereof a laval nozzle through which air flows at the speed of sound even under low-speed low-load engine operating conditions. The laval nozzle is connected to a carburetion chamber which is elongated straightly in the direction downstream of the laval nozzle. A fuel injection valve is disposed in the primary passage at a place upstream of the laval nozzle. A pressure control valve is adapted so that a vacuum higher than a predetermined value is developed in the carburetion chamber at any time. A primary throttle valve and a secondary throttle valve are interconnected so that the secondary throttle valve is held closed unless the primary throttle valve is fully open. The primary throttle valve is movable in response to an engine operating condition.
