Provided is a fuel fill opening structure which includes: a holder which forms a fuel supply passage; a main on-off valve which is arranged in the holder, and opens or closes the fuel supply passage; a drain opening which is drilled on an upstream side from the main on-off valve of the holder; a drain flap which is pivotably arranged at a position to come in contact with a fuel gun inserted into the holder; and a drain on-off valve which pivots along with pivot of the drain flap to open or close the drain opening. A first pivot amount of the drain flap, which is pivoted by being brought in contact with the fuel gun, is set to be larger than a second pivot amount of the drain on-off valve required to close the drain opening.