To correspond to a case that a set space is extremely restricted by miniaturizing a flow controlling means comprising a throttle valve for output control and an air valve for controlling scavenging air flow and shortening a mixture passage and an air passage.

The throttle valve 22 of the mixture passage 6 connected to a crank chamber 3 and the air valve 25 of the air passage 9 connected to a scavenging passage 8 are integrally formed by engaging one cylindrical valve element 27 having a throttle through hole 23 and an air through hole 26 with one body 28. The mixture passage 6 is straight in the body 28, the air passage 9 is directed obliquely upward on the downstream side of the valve element 27, and the upstream sides of the valve element 27 of the passages 6 and 9 are formed with one common inlet passage 30.