To prevent fuel from introducing into an air vent passage by overflowing the fuel from an oil-feeding port cylinder before the fuel oil level reaches the top opening of the air vent passage when fuel is excessively supplied to a fuel tank so that the fuel rise to the oil-feeding port cylinder.

In this air venting device of a fuel tank for an engine, the upper ends of the air vent passages 22-24 penetrating through the fuel tank T downward and communicating with the atmosphere are opened in the inner wall of the oil-feeding port cylinder 20 sealed by a tank cap 21 of the fuel tank T. The upper ends of overflow passages 36 and 37 extending to the downward side of the fuel tank T are opened at positions lower than the upper ends of the air vent passages 22-24 in the inner wall of the oil-feeding port cylinder 20.