Internal combustion engine and straddle-type vehicle equipped with the engine

In a single-cylinder internal combustion engine fitted with a knock sensor, the temperature rise of the knock sensor is suppressed, the reliability of the knock sensor is improved. An engine (10) has a crank case (11) accommodating a crankshaft (17), a cylinder block (12) connected to the crank case (11) and having a cylinder (15) formed therein, a cylinder head (13) connected to the cylinder block (12), a sensor mounting boss (40) formed on the crank block (12), a knock sensor for detecting knocking, mounted to the boss (40), a fan for guiding air to at least the boss (40), and an air shroud (30).