A crankcase emission control device of an internal combustion engine comprises a gas/liquid separator that separates an oil mist from blow-by gases flowing therethrough. A first passage is defined in both a crankcase and a cylinder head to connect an interior of the crankcase to a valve rocker cover chamber defined by the cylinder head. A second passage extends from the valve rocker cover chamber to the gas/liquid separator. A third passage extends from the gas/liquid separator to an intake system of the engine. For reducing the height of the entire construction of the engine, the gas/liquid separator is integrally formed by the cylinder head at a position below intake ports of the cylinder head.