Filing port structure for fuel tank

A filling port structure for a fuel tank includes a filling port member (16) that defines a filling port, an open/close valve (20) that opens and closes the filling port, a rotary member (22) that attaches the open/close valve (20) to the filling port member (16) in a manner such that the open/close valve (20) is rotatable between an open position and a closed position, a locking member that is provided in the filling port member (16) and is movable between a locking position to lock the open/close valve (20) in the closed position and an unlocking position to unlock the open/close valve (20), an unlocking member that moves the locking member to the unlocking position when pressed by a fuel gun, and a retention portion that holds the locking member in the unlocking position at least when the open/close valve (20) is in the open position.