The device comprises, on one hand, a degassing tube (5) having one end (5a) which opens onto the upper inner part of the tank, markedly lower down than an upper wall (2) of the tank, and, on the other hand, a pipe (8) connecting the inner part to the exterior of the tank through a valve (V) carried by the wall (2), the valve being of the normally open ball (19) and float (10) type, the float (10) extending sufficiently downwardly to be raised by the fuel (7) when the latter reaches said end (5a), the lower end (10B) of the float (10) being lower than the end (5a) of the tube (5), and means are provided to ensure that this raising closes the valve (V) in a partly opened state. This device ensures simultaneously a permanent communication of the inside of the tank (1) with the atmosphere, a guarantee of an integrated expansion volume in the tank, and the prevention of flow of fuel in the event of an overturning of the vehicle.