An apparatus is for inhibiting fuels from flowing out of fuel tanks, and includes a housing and a floating valve. The housing has an evaporator opening, and a major through hole communicating the inside of the housing with the outside and having an opening width sharply reducing from wide to narrow in the direction from the bottom end to the top end. The apparatus detects that the fuel tank is filled up with a liquid fuel by increasing a gas pressure within the fuel tank when the liquid-fuel level is placed adjacent to the top end of the major through hole, and closes the evaporator opening with the floating valve when the level of the liquid fuel rises abnormally.