A fuel feed system of an engine is provided with a gas-fuel separating unit for separating oil mist generated in an engine case from air with a labyrinth, and an auto fuel cock is operated by pressure pulsation of the air from which the oil mist is separated by the gas-liquid separating unit. Thus, infiltration of the oil mist into the auto fuel cock is suppressed to the minimum, and a malfunction of the auto fuel cock caused by accumulation of the oil can be prevented. Additionally, a breather passage for feeding the air, from which the oil mist is separated by the gas-liquid separating unit, to a breathing unit is connected the auto fuel cock via a negative pressure tube. Thus, it is unnecessary to provide a specific passage for transmitting the pressure pulsation of the air in the engine case to the auto fuel cock.