In a method for filling a bottle, a balloon body extends through an axial through bore of a cap and is connected seal-tightly with an upper circumferential rim to the cap above a radial opening of the cap. An elongate support associated with the balloon body is connected to the cap. Balloon and elongate support are inserted into the bottle. The cap is positioned at a spacing above the bottle neck to form a ring-shaped intermediate gap. Air contained in the bottle interior is forced out through the intermediate gap by inflating the balloon body with an expansion medium. Subsequently, the cap is fluid-tightly fitted on the bottle neck. The expansion medium is allowed to escape from the balloon body and liquid is supplied through radial and axial openings of the cap into the space between outer surface of the balloon body and inner bottle wall.