PURPOSE: To shorten combustion time and aim at lower NOx and a better specific fuel consumption by providing an auxiliary pipe that is separate from a main pipe that runs in the direction of the intake port of a cylinder and setting at a special position an opening end of the auxiliary pipe in order to develop effective swirl.

CONSTITUTION: A main pipe 2 that is opened and closed by an intake valve 8 opens in a cylinder 5 of a four-cycle engine, and an auxiliary pipe 7 provided separate from the main pipe 2 also opens. Here, the opening end 7a of the auxiliary pipe 7 is right behind the intake valve 8, and further its direction is made tangential to the cylinder 5. In more detail the above mentioned opening end 7a is in a circle 0 drawn at the spark point P of the ignition plug 6 as center with the diameter equal to the inner diameter of the cylinder 5 and is formed in the space within 1mm in reference to the plane S containing the point a on the side nearer to the cylinder wall of the two contact points made by the tangential lines drawn from the spark point P of the spark plug 6 to the circle 02 that is the contact circle of the valve seat 9 and the center line of the valve stem 8a.