PURPOSE: To improve the mixing of the air and the fuel, by utilizing the suction, exhaust valves at the top of the cylinder to form a sub-combustion chamber at the position in a piston head coinciding with the main combustion chamber thereby producing a strong spiral flow and providing the function for directly injecting the spiral flow.

CONSTITUTION: A main combustion chamber 7 comprising a tubular recess 6 is formed at the position facing with the valve head 5' of a piston head 3' while a tubular member 8 opening to the piston 3 side is projected at the tip of the valve head 5' thus to form a sub-combustion chamber 9. While a plurality of holes 10 are made circumferentially in the thick section 8' of the tubular member 8 and a groove 11 conductable to said hole 10 is formed around said recess 6. Consequently under compressing stroke the air in the cylinder 1 is compressed into the tubular member 8 then directed to the circumferential direction through the hole 10 and injected to produce the spiral flow. When the piston 3 rises to fit the recess 6 and the tubular member 8, the main and sub-combustion chambers 7, 9 are conducted to produce the highly compressed spiral flow resulting in the improvement of the mixing of the air and the fuel.