An oil mist separation mechanism of an internal combustion engine includes an oil mist separation portion being integrally provided with an internal combustion engine main body at an outer surface thereof, the oil mist separation portion for separating an oil mist included in a blow-by gas sent from the internal combustion engine main body, and an oil returning portion being connected to the oil mist separation portion, the oil returning portion returning an oil separated by the oil mist separation portion to an oil stored at a lower portion of the internal combustion engine main body.