A capless refueling mechanism includes a fill opening side shut-off valve disposed near the fill opening, and a fuel tank side shut-off valve disposed nearer to a fuel tank than the fill opening side shut-off valve. The refueling auxiliary device has a guide pipe section and a funnel section, and pressing of the fill opening side shut-off valve and the fuel tank side shut-off valve via an end of the guide pipe section causes a valve element to be moved away from a seat portion and insertion of a nozzle into the funnel section allows refueling. The outer circumferential surface of the guide pipe section is provided with an enlarged diameter section that regulates a valve opening degree of the valve element of the fuel tank side shut-off valve.