A lock pin is arranged so as to be projected from a side wall of a concave region, in which a fuel filler port of a vehicle is arranged, toward the inside of the concave region. In addition, a retainer placed around the lock pin on an inner side of the side wall of the concave region and a protector supported by the retainer so as to be placed in a region close to the lock pin are provided. The protector has a detent pin to prevent the retainer from rotating relative to the lock pin.