PURPOSE: To improve combustibility of a mixture through its centrifugal force and high temperature of an exhaust port, by forming peripheral stepped part to the upper part in a combustion chamber, firstly causing a swirl internally of the sucked mixture, and then leading the mixture to its lower side.

CONSTITUTION: A mixture from an intake port 6 is led firstly to a guiding surface 11 of one side and then to a guiding surface 12 of the other side of a space in a stepped part 10, to cause a swirl, when a liquid state fuel component is led in a peripheral direction by centrifugal force to become a rich mixture, while vaporization of fuel is promoted by high temperature in the side of an exhaust port 5 to improve quality of the mixture. This swirl is further led to a lower side through the stepped part 10, and a rich mixture is formed also in the periphery of the stepped part. Then secondary air is led from an introducing hole in the end of an intake stroke, and upward movement of a piston obtains a rich mixture layer A in the lower side periphery, lean mixture layer B in the top inside further a lean mixture layer C in the lower of said stepped part 10. Then a flame in a recessed part 13 from firing causes propagation such as A→B→C, thus good combustion of the three layers can be obtained.