To secure sufficient intake air quantity so as to enable combustion efficiency to be improved by forming a sub-port in a taper shape so as to increase the flow speed of intake air even if its passage sectional area is decreased from upper course side to lower course side.

In an internal combustion engine formed by partitioning the suction port of the intake air manifold 2 of the internal combustion engine in a main port 3 in which a flow control valve 5 is provided and a sub-port 4 having its passage sectional area smaller than that of the main port 3 and formed in a tip tapered shape, a forcibly feeding means 25 is provided for feeding intake air to the entrance side of the sub-port 4 by the action of exhaust gas pressure.