PURPOSE: To prevent freezing in a carburetor by providing a cooling air discharge guide case along the peripheral wall of at least one of a cylinder head and a cylinder, placing a cooling air discharge passage in the cooling air discharge guide case, and guiding the cooling discharged air discharged from the cooling air passage to the above cooling air discharge passage.

CONSTITUTION: A cooling air passage 3 for cooling one of a cylinder head 1 and a cylinder 2 is provided, and a cooling air discharge guide case is disposed along the peripheral wall of the cylinder head 1 or the like, thereby forming a cooling air discharge passage 5. A cooling discharged air 6 discharged from the cooling air passage 3 is introduced into the cooling air discharge passage 5. An exhaust pipe 9 interposed between an exhaust port 7 and an exhaust muffler 8 is stored in the cooling air discharge passage 5, a discharge air lead- out pie 11 is derived from a passage outlet 10 of the cooling air discharge passage 5, and the discharge air lead-out pipe 11 is directed to a carburetor 14. Thus, cooling discharge air 17 heated is sprayed to a carburetor 14 so as to prevent freezing in the carburetor 14.