A device for sealing a rotary shaft (17), which is supported in bearings (18, 19) in an air-tight casing (20), which extends into a container (10), in which overpressure is maintained. The device comprises a seal holder (22) attached in the casing (20) with a mechanical shaft seal (21). The casing has openings (23) to render possible access to the side of the shaft seal (21) located in the casing (20). A detachable air-tight cover (24) is provided to close said openings (23) at operation.