An apparatus is for inhibiting fuels from flowing out of fuel tanks, and includes a housing having an evaporator opening, a floating valve, a seat, a cylinder having a peripheral wall and a lateral hole formed in the peripheral wall and communicating the inside of the cylinder with the outside, a bottom hole formed in the seat and communicating the inside of the cylinder with the outside, and a flow passage extending from the bottom hole to the lateral hole, flow passage in which a part of the peripheral wall of the cylinder intervenes. Alternatively, the apparatus include a flow passage extending from the bottom hole to the evaporator opening and constricted partially by the seat and a bottom of the floating valve. Thus, gases are less likely to float the floating valve upward, but liquids are likely to float it upward.