PURPOSE: To prevent the generation of a blow-by phenomenon and fuel loss by installing an air passage which opens in the position adjacent to a scavenging port inside a scavenging passage and installing a check valve.

CONSTITUTION: A spark plug 27 is mounted on the upper surface of a combustion chamber 25 in the upper part inside a cylinder 15, and an exhaust port 29 and a scavenging port 31 are opened at the end point of the falling down of a piston 17. An air passage 43 is opened in the position adjacent to the scavenging port 31 inside a scavenging passage 41 which connects the scavenging port 31 to a crank case 13. A check valve 49 which communicates only in the direction of the scavenging passage 41 is installed in the air passage 43. Direct exhaust of a mixed gas ejected from the scavenging port 31 is prevented and fuel loss is prevented for enhancing a fuel consumption rate.