PURPOSE: To obtain an intense swirl air flow by a construction wherein an air flow guide wall is formed in the peripheries of an air inlet value a stem and a valve stem guide so as to extend from the upper surface of a spiral passage thereby preventing the swirl flow caused by the spiral passage from being caught around the air intake valve stem or the valve stem guide.

CONSTITUTION: The helical port is defined by a spiral passage 20 formed in the terminal end of an air inlet passage 18. The air introduced into a cylinder 10 is given a swirl motion by the passage 20. In the peripheries of an air inlet valve stem 14 and a valve stem guide 16, a circumferential wall 26 extends downwardly from the upper surface 24 of the passage 20. The circumferential wall 26 extends gradually downwards from QL line so that it becomes largest near QI and QH lines and smaller as it approaches QA line, and zero at QA line, Further, the circumferential wall 26 is formed in the angular range of 270° in the periphery of the valve stem 14, whilst a partition wall 28 is formed in the range of the remaining 90°. This arrangement enables the air drawn into the cylinder to be guided by the circumferential wall 26 and prevents the air from being dragged around the valve stem 14 and the valve stem guide 16.