To provide an internal combustion engine capable of suppressing oil deterioration by efficiently removing unburned fuel in a crankcase.

This internal combustion engine is provided with a gas recirculation mechanism for recirculating gas G in the crankcase 25 to an intake passage 11. The gas recirculation mechanism has a pump 103 for reducing pressure in the crankcase 25 by sucking the gas G in the crankcase 25 provided in a gas recirculation passage 104 and an oil mist separator 100 provided between the pump 103 and the crankcase 25 to separate oil components contained in the gas G sucked by the pump 103.