To provide a cooling structure of an air-cooled engine capable of avoiding increase of size of the engine and ensuring degree of freedom in design of the engine and a vehicle body when adding a water-cooled system.

In this cooling structure of the air-cooled engine constituted by forming many cooling fins 50, 51 in cylinder blocks 17, 18 and cylinder heads 19, 20, a cooling jacket 52 is formed between an exhaust port of the cylinder head and a mating surface on a cylinder block side to circulate cooling water between the cooling jacket 52 and a radiator 54.