An automotive internal combustion engine having main and auxiliary combustion chambers for each of the cylinders and combined with a carburetor is provided with an air-fuel mixture induction system which comprises an intake manifold providing communication between the carburetor and each of the main combustion chambers, characterized by passageways leading to the auxiliary combustion chambers and open into the intake manifold or the intake ports for the main combustion chambers through an opening in the main tube portion of the intake manifold, openings respectively formed in the runner portions of the intake manifold or openings formed in wall portions defining the intake ports for the main combustion chambers.