The invention relates to an apparatus and a method for filling product, in particular liquid foodstuffs, into containers. Whilst the containers pass through a working chamber of the apparatus from an inlet to an outlet side, the containers passing through the working chamber are acted upon by a sterile fluid, in particular sterile air, in order to avoid contamination. In order to improve the sterile atmosphere in the working chamber, the invention proposes that an external line extends through the working chamber having a plurality of gas openings for the introduction of sterile air into the working chamber and that an internal line extends into the external line having a plurality of openings for spraying a cleaning medium. An annular chamber extends in a longitudinal direction between the internal and the external line having a closed end and an inlet for supplying the sterile fluid into the annular chamber which is configured to predefine a flow direction in the annular chamber for the sterile fluid starting from the inlet towards the closed end. The cross- sectional area of the annular chamber varies in the longitudinal direction, at least in sections thereby allowing to influence the static pressure and the flow velocity of the sterile fluid in the annular chamber such that a constant pressure distribution along the length of the outer line and thereby an even distribution of sterile air emerging from the gas openings into the working chamber is achieved.