PURPOSE: To improve a radiating efficiency and a mechanical strength, by a method wherein cooling fins, which have their respective lengthes being different between the ones at a high-temperature portion and a low-temperature portion, are constituted such that the longer a fin is, the wider taper it has, and as the fin decreases in the length, the taper reduces gradually.

CONSTITUTION: A cylinder of an air-cooled engine is constituted such that a portion 3a, which is positioned nearmost to a combustion chamber, has the longest cooling fin 3, and following fins 3b, 3c... become shorter in order as named, and a fin 3f, which is positioned nearmost to a crank case 6, is the shortest. A cooling fin 4, which is longer than the longmost fin 3, is formed at a cylinder head 2. The cooling fins 3 and 4 are formed such that the longer it is the wider a taper, which is formed with the two sides, becomes. For example, the tapers are formed so as to be θ1=6°, θ2=5.5°, θ3=5°, θ4=4.5°, θ5=4°, θ6=3.5°, θ7=3°, θ8=3°, where θ1 is a taper of the fin 4, and θ2Wθ8 are the tapers of the fins 3aW3f, respectively.