An air suction system for an internal combustion engine in which one of two air intake port sections is provided with an air intake control valve to provide enhanced atomization and ensure a sufficient airflow. The air suction system, according to an embodiment of the invention, has a passage cross-section of a first injection passage portion which is greater in area than that of a second injection passage portion. The first and second injection passage portions respectively communicate with the first and second intake port sections. In a variation, the first and second injection passage portions have passage cross-sections adjacent to the first and second nozzle hole portions of smaller cross-sectional area, and the first and second injection passage portions have passage cross-sections adjacent the first and second intake port sections of larger cross-sectional area. In a further variation, the first and second injection passages are formed with taper-shaped configurations such that the area of each passage increases progressively in a direction toward the first and second intake ports and away from the air intake control valve.