To provide an apparatus for treating a fibrous web, in which a sealing device can be adapted to various operating states, in order to permit optimum conditions in particular in a relevant air-based device, e.g. for transferring, spreading the web run during operation and in the case of different supporting belts, e.g. wires.

This apparatus for treating a fibrous web such as in particular a paper and/or board web is provided; wherein the fibrous web, on its own or together with at least one supporting belt, e.g. a dewatering belt, press belt, mesh belt and/or the like, is guided over at least one moving or stationary supporting surface; on the side of the fibrous web opposite to such a supporting surface, at least one sealing device is provided, which includes a flexible sealing blade which is held in a holder and can be acted on by a pressing device, via which the distance between the supporting belt and the sealing section of the sealing blade interacting with the supporting belt can be set variably.