PURPOSE: To improve starting performance at the time of initial starting by forming a bypass passage for leading directly air-fuel mixture discharged from a crank chamber into an intake port, and providing a switching valve for leading the air-fuel mixture discharged from the crank chamber into the bypass passage.

CONSTITUTION: A passage part 31 for leading air-fuel mixture discharged from a crank chamber 4 into the main body of a supercharging chamber 30 is formed in one body on a connecting part side toward the crank chamber 4 in the supercharging chamber 30. A bypass passage 32 for leading air-fuel mixture discharged from the crank chamber 4 directly into an intake port 15 without passing through the main body of the supercharging chamber 30 is formed between the passage part 31 and the intake port 15. A switching valve 33 for leading air-fuel mixture discharged from the crank chamber 4 into the bypass passage 32 by switching the intake port 15 to its closing condition is provided in the intake port 15 being freely opened and closed. Since air-fuel mixture is led from the inside of bypass passage 32 directly into the intake port 15 at the time of initial starting, it is possible to improve starting performance.