PURPOSE: To reduce blowing-by of scavenging air at the time of low speed or partial load driving, so as to improve fuel consumption by arranging an air port in the middle of exhaust and scavenging ports, and by opening the air port in the vicinity of a bottom dead center of a piston.

CONSTITUTION: When a piston 16 is elevated by the rotation of a crank shaft 14 through a connecting rod 18, an air-fuel mixture is blown in a crank chamber 22 from an intake path 24. When the piston 16 is lowered, the air-fuel mixture pre-loaded in the crank chamber 22 is flown in a combustion chamber, from scavenging air ports 36, 38. By the scavenging air, the gas already burned in the combustion chamber is exhausted from an exhausting port 26. At the time of low speed or partial load driving of an eingine, a negative pressure of cyclic pressure variation due to the pulsation of the exhaust that works at the final stage of exhausting process, or to inertia, is worked on an air port arranged in the middle of each port 26;36, 38, and air is thus inhaled in the combustion chamber.