A fueling portion structure of a fuel tank can reliably close a fueling port, and at which pulling-out of a fueling gun from the fueling port is easier. A center of a rotation shaft and a contact position when valve is closed, that a spring for the flapper valve contacts when a flapper valve is at a closed-valve position, are connected, and a reference segment when the valve is closed is assumed. The center of the rotation shaft and a contact position when the valve is open, at a time when the flapper valve is at open-valve position HP, are connected, and a reference segment when the valve is open is assumed. A spring shaft is provided further toward the contact position when valve is closed side than a bisector of an angle formed by the reference segment when valve is closed and the reference segment when valve is open.