To efficiently cool high temperature portions such as an exhaust port or an ignition plug in OHC air cooled engine.

In this engine, a cylinder head 10 is spaced from a housing 13a housing a valve system through a valve support part 10b. The valve support part 10b is divided into an intake side and an exhaust side through a blowing introduction part 10e. An ignition plug 12 is faced to a portion in the vicinity of the blowing introduction part 10e. When cooling air is introduced to the blowing introduction part 10e, high temperature portions such as an exhaust port 10d, an ignition plug 12 positioned on the base of the exhaust side valve support part 10b are directly cooled by the cooling air. Cooling performance is thus improved.