A fuel-escape prevention device 10 for sealing in the top of a tank 30, which device 10 comprises a first housing component 11 having an evaporation passage 11b and a second housing component 12 fused with the first housing component 11 and containing a float valve member 16 capable of cutting off communication between the tank 30 and the evaporation passage 11b via engagement with a valve hole 12g in a partition 12h between the two housing components 11, 12. The second housing component 12 also includes an external circumferential shoulder 12a passable through a seal 31 upon installation in the tank 30. The float valve member 16 may comprise a disk 16d and a number of ribs 16a to guide it in the second housing component 12. Preferably, a pressure relief valve 20 is provided in the partition 12h. The first housing component 11 may have a lower end of greater diameter fused with a step of reduced diameter on the second housing component 12. Alternatively, the second housing component 12 may include an upper projecting portion of larger diameter fused with a lower part of the first housing component 11.