A fueling-error prevention device is provided in an inside of a fuel flow path between a fueling opening and a fuel tank. A flap valve is provided on one of the first and the second sliders which are disposed opposite each other with the fuel flow path positioned therebetween. Accompanied by an insertion of a fueling nozzle having a predetermined diameter, the fueling nozzle abuts against either the first or the second slider so as to move one slider in a separating direction. Thus, the other of the first or the second slider is allowed to move in the separating direction, and due to the movement of the other slider in the separating direction by abutment of the fueling nozzle, the flap valve is allowed to be opened.