In an internal combustion engine (10), oil in an oil pan (21) is pressure-fed to lubrication sites through operation of an oil pump (22). After being used for lubrication of the lubrication sites, the oil runs down and collects in the oil pan (21). The internal combustion engine (10) is provided with a blow-by gas process device (40) including a communication passageway (48) that connects a portion of the intake passageway (12) on the downstream side of the throttle valve (13) in the intake flow direction and an interior of the crankcase (15) in communication. A pressure chamber (47) to whichan end portion of the communication passageway (48) is open is defined in the interior of the crankcase (15). Oil scattered in the interior of the crankcase (15) is introduced into an interior of thepressure chamber (47) before mixing with the oil collected in the oil pan (21).