PURPOSE: To favourably scavenge the overall inside of a combustion chamber by arranging an additional exhaust valve in the central part on a cylinder head inwall surface as well as arranging an exhaust valve in one side peripheral part on the cylinder head inwall surface.

CONSTITUTION: In one side peripheral part on a cylinder head inwall surface 3a, a pair of exhaust valves 9, 10 is arranged. In the other side peripheral part of the cylinder head inwall surface, a pair of intake valves 11, 12 is arranged. Additionally, in the central part of the cylinder head inwall surface 3a, an additional exhaust valve 13 is arranged. Fresh air flows into a main chamber 4 through the exhaust valves 9, 10, 13 and each of the intake valves 11, 12 positioned on the opposite side. In this case, fresh air flows down along a cylinder bore inwall surface 1a below the intake valves 11, 12. Thereafter, fresh air flows upward along the cylinder bore inwall surface 1a below the exhaust valves 9, 10. As the exhaust valve 13 is provided, combusted gas existing in the central part of the main chamber 4 is scavenged by fresh air X flowing into from the intake valves 11, 12. Consequently, the overall inside of the main chamber 4 is favourably scavenged.