Air-cooled internal combustion engine

The engine has a valve gear mechanism provided above a combustion chamber and a cooling air passage (32) extending in fore-and-aft direction between the chamber and a valve gear mechanism along an exhaust and an intake port. A laterally extending cooling air passage (35) is in communication with the passage and includes an ignition plug (27). A cooling fin curved from the former passage extends towards the latter passage. The exhaust port is inclined with respect to the fore-and-aft direction to broaden a front opening of the former passage.