PURPOSE: To stabilize the combustion and improve the thermal efficiency of the titled device by a method wherein a sub-scavenging passage, other than a main scavenging passage, is provided to feed the gas (or air) from a gas (or air) pressurize feeding device, and the scavenging timing from each scavenging passage is mutually delayed.

CONSTITUTION: When the titled device is applied to a single cylinder air cooling two cycle engine using the propane gas as the fuel, a main scavenging port 10, an exhaust port 11 and a gas feeding port 29 as a sub-scavenging port are formed. Said main scavenging port 10 is opened when a piston 5 moves down inside of a cylinder 3, and communicated with a crank chamber 2 via a main scavenging passage 9. A gas feeding 29 is communicated with a diaphragm type gas pressurize feeding device 15 which is driven by a cam 24 fixed to a crank shaft 7 via a gas feeding passage 28. After the sufficient scavenging is performed during the scavenging stroke, the propane gas is fed into said cylinder 3 through said gas feeding port 29 by said gas pressurize feeding device 15 within the time from the down dead center to just before the closing of said exhaust port 11.