PURPOSE: To equalize the air-fuel ratio of a mixture accelerating the rising up combustion as well as to improve the high speed output performance reducing fuel consumption by a method wherein a swirl guide is protruded beyond the ceiling of a combustion chamber so that the swirl guide may be inserted between two suction valve ports.

CONSTITUTION: When a suction stroke is started, the second suction valve 182 is opened firstly, therefore swirl is produced due to the two factors, i.e. the first that a mixture flowing from the second suction valve port 92 to a combustion chamber 6 is almost directed in the direction of the tangential line around the combustion chamber 6 by the second branch port 132 and the second that the flowing to the first suction valve port 91 side is interrupted by the first guide surface 8a of a swirl guide 8 flowing in the direction of the second suction valve port 92, an ignition plug 12 and an exhaust valve port 10 in this order. Next when the first suction valve 181 is opened, the mixture flowing from the first suction valve port 91 to the combustion chamber 6 is led by the second guide surface 8b of the swirl guide 8 to produce swirl and the air-fuel ratio of the other mixture in the combustion chamber 6 is equalized by intensifying the swirl while flowing together with the mixture preliminarily flowing from the first suction valve port 91. Through these procedures, the fuel consumption may be reduced improving the output performance.