PURPOSE: To obtain a large swirl ratio during low speed operation of an engine and a small swirl ratio during high-speed operation, and ultimately to improve the combustion efficiency of an engine by forming a stepped edge at the connection part between a suction manifold and a suction port.

CONSTITUTION: A stepped edge 16 is formed at the connection part between a suction manifold and a suction port. When an engine 10 is operating at a low- speed, the flow rate of the air sucked from the suction manifold 13 is relatively slow. Thus the negative pressure of an area 17 created due to the stepped edge 16 is small and the air flows from the suction manifold 13 to a helical suction port 12 without being changed so much by the stepped edge 16. This is indicated by the singly-dotted line in the diagram. As a result, the swirl ratio becomes larger. On the other hand, when the engine is operating at a high speed, the negative pressure of the area 17 created due to the stepped edge 16 becomes larger and the flow is bent by the stepped edge 16. In other words, the air flows along the side wall 19 of the helical suction port 12 as shown by the doublly- dotted line and the swirl ratio becomes smaller.