A fuel outflow preventing apparatus of a fuel tank for a vehicle has an object to prevent fuel outflow from the fuel tank when the vehicle is inclined or turned over. Therefore, this apparatus comprises a canister, a float valve provided in an air chamber of the fuel tank, which float valve is closed when the level of fuel in the fuel tank is equal to or higher than a predetermined level and when the vehicle is inclined or turned over, a fuel outflow preventing valve provided in an upper portion of the air chamber of the fuel tank, which fuel outflow preventing valve is closed when the vehicle is inclined or turned over, a first evaporation passage for communicating the float valve with the canister, a second evaporation passage for communicating the fuel outflow preventing valve with the canister, and opened/closed valve device provided in the second evaporation passage.