A fuel cutoff valve of the invention attached to an upper portion of a fuel tank includes a casing, a first part supported by multiple legs, a second part, and a spring. The second part is located below the first part in a vertically movable manner and has multiple air holes formed in a top plate to communicate with a buoyancy chamber. When the fuel level in the fuel tank rises to submerge the second part and reach the first part, the first part rises by its buoyancy to close a connection conduit. In the event of an abrupt rise of the fuel level at a high increase rate, for example, by a sudden turn of the vehicle, the second part rises by the increased buoyancy of the buoyancy chamber by the air retained in the buoyancy chamber and presses the first part up to close the connection conduit. The fuel cutoff valve of the invention promptly closes in response to a heavy ruffle of the fuel level by a sudden turn of the vehicle, so as to prevent the outflow of the fuel from the fuel tank.