A device prevents fuel mixing, for example, a different kind of fuel from being pumped into a gasoline vehicle or a diesel vehicle by mistake. The device can prevent unsuitable fuel from being pumped into a vehicle and can preclude a loss of fuel or a fire due to poor operation of a gas pump by a person who fuels the vehicle, by implementing a new type of mechanism for preventing fuel mixing which includes a lock that is disposed at the fuel-tank door of a vehicle and turned on/off in accordance with the inner diameter of a nozzle and a cap that can close the nozzle when a nozzle for an unsuitable kind of fuel is inserted.