A fuel cutoff valve comprises a casing having a seal portion disposed facing a connecting passage and projecting in annular configuration towards a valve chamber; and a float mechanism having a float which rises and falls due to increasing and decreasing buoyancy according to the fuel level within the valve chamber. The float comprises a flexural depression-enabling recess situated on its upper surface at a location in opposition to the seal portion, for enabling the seat portion to undergo flexural depression when pressed by the seal portion. The casing has a regulating portion on the outside perimeter of the seal portion, for regulating the level of elastic deformation at which the seat portion elastically deforms into the flexural depression-enabling recess.