A helically-shaped intake port comprising a helical portion and an inlet passage portion which is tangentially connected to the helical portion and extends so as to be slightly curved. The intake valve is arranged at the outlet open end of the helical portion. The first side wall of the inlet passage portion, which is located near the axis of the intake valve, has on its upper portion an inclined wall portion which is arranged to be directed downwards. The inlet passage portion has an open inlet end formed on the flat side wall of the cylinder head. The inlet passage portion is so arranged that the longitudinal axis thereof obliquely intersects with a straight line passing through the center of the valve head of the intake valve and extending parpendicular to the outer side wall of the cylinder head. The inlet end of the inlet passage portion is arranged at a position remote from the straight line so that the inlet end does not include the straight line.