Abstract: The object of the present invention is to concentrate the intake air flowing into a combustion chamber of an internal combustion engine near the ignition point of a spark plug, and to form a strong swirl effect in this area.A projection 16 is formed on an end surface 5a of a piston 5. Projection 16 is positioned near an intake port disposed on one side of the upper surface of a combustion chamber. A cavity 17 is formed at a position closer to an exhaust port than projection 16. An ignition point of a spark plug is disposed above cavity 17. The intake port points toward the outer perimeter of the combustion chamber. The portion of projection 16 on piston end surface 5a that lies in the direction of the intake port is cut away. An intake flow passage 19 is formed on end surface 5a of piston 5, extending from a cut-away section 18 toward the exhaust port via the outer perimeter of piston end surface 5a.