A composite control valve is constructed by a valve housing, a negative pressure working chamber defined in the valve housing to communicate with a negative pressure generating section in the engine, a first control valve operable to be opened and closed with generation and extinction of a negative pressure in the negative pressure working chamber, and a second control valve operable to be opened and closed with the generation and losing of the negative pressure in the negative pressure working chamber. The first control valve is incorporated into an air vent system which provides communication between an upper space in a fuel tank and the atmosphere, and the second control valve is incorporated into a fuel passage system which provides communication between a portion of the fuel tank below a fuel oil surface and a fuel supply section in the engine. At least a portion of the air vent system is constituted by an inner air vent pipe disposed within the fuel tank, and a gas-liquid separating device is provided in an opening of the inner air vent pipe into the upper space. Thus, upon stoppage of the operation of the engine, not only the fuel passage system but also the air vent system leading to the upper space in the fuel tank can be blocked simultaneously, to prevent release of evaporated fuel generated in the fuel tank to the atmosphere.