A closing mechanism for a filling tube of a fuel tank in an automobile, including: a first housing portion adapted to be fastened in an opening of the body of the automobile, a closing slide member which supported for linear or pivotal, movement transverse to the axis of the opening, the closing slide member and a portion of the housing portion together form a funnel-like guide means such that upon a movement of a fuel nozzle towards the guide means the closing slide member is moved into an open position whereby the nozzle can be moved further into the opening, a second housing portion having a first end connectable to the first housing portion and a second end attachable to the end of the filling tube, the second housing portion having a second opening aligned with the first opening, and a closing flap for the second opening.