A closed crankcase ventilation system in which blow-by gas is introduced into an oil separator through a blow-by gas blowout opening provided in a truss structure of a cylinder block. A diaphragm is established in the truss structure of the cylinder block to close a portion of an oil hole portion returned from a cylinder head on the top thereof, thus preventing oil from flowing backward. An oil drain portion returns oil separated in the oil separator to an oil pan, and a gas recycling portion supplies blow-by gas separated in the oil separator to an intake manifold.