The invention relates to a closure membrane (2) with a closure head (5) and a retaining border (3), the closure head being connected to the retaining border via a connecting wall (4) adjoining the outer border, with the entire arrangement being of essentially cup-shaped design, the closure head preferably being of a thickness which increases outwards from the centre. The connecting wall is attached to the closure head via a connecting web (51) which is of a lesser thickness than the border region of the closure head and which in the rest position of the membrane projects radially inwards from the connecting wall.