NOVELTY - Servicing device (2) comprises servicing unit (3) and servicing section, where device is used in biogas plant having fermenter tank with fermentation section (5) and gas section (6) at interior space, where tank is closed gas-tight by gas closing wall disposed at tank, sealing device (9) is provided to effect gas-tight separation of servicing section from gas section. The sealing device has retention wall of flexible material and when installed is movable between inoperative (10)/sealing positions. The servicing section is linked to gas section to be gas-permeable in inoperative position.    USE - As servicing device for biogas plant (claimed).    ADVANTAGE - The device exhibits ease of operation and ease of maintenance while minimizing biogas losses when carrying out maintenance. In the sealing position, the servicing section is sealed gas-tight from the gas section by the flexible retention wall. The flexible retention wall allows maintenance on an agitator with a variable fermenter mass filling level. The device provides ease of maintenance. The flexible length of the retention wall allows a flexible adjustment to the currently prevailing conditions. Concurrently the sealing device is provided at the servicing device including in the inoperative position and does not require removal every time after maintenance. In the inoperative position, the sealing device is mostly provided above the liquid level and is at least not immersed in the liquid. This prevents a greatly increased flow resistance. The device avoids use of converting and installing component for every maintenance. The device allows uncomplicated and easy maintenance on an agitator or an agitator device located in the servicing section while at the same time prohibiting the biogas located in the gas section of the fermenter tank from escaping to the exterior through the servicing section of the servicing unit. This allows uncomplicated maintenance involving minor losses of gas. The scouring period during which fresh air is fed to the servicing section to avoid explosive gas mixtures is kept very brief. After a performed maintenance, the biogas plant can be restarted immediately without removing any components which on the whole causes considerably lower costs and losses. The sealing device may be telescopically extendable and may e.g. comprise slightly conical and flexible or gas-tight, flexibly interconnected cylinder segments which are telescoped. This does not require for the sealing body to be shaped cylindrically but its cross-section may be rectangular or polygonal.    DETAILED DESCRIPTION - A servicing device (2) comprises a servicing unit (3) and a servicing section, where the device is provided to be employed in a biogas plant, where the biogas plant has at least one fermenter tank with a fermentation section (5) and a gas section (6) at an interior space of the fermenter tank which fermenter tank is closed gas-tight by at least one gas closing wall disposed at the fermenter tank, at least one sealing device (9) is provided which is suitable to effect a gas-tight separation of the servicing section from the gas section, where the sealing device includes at least one retention wall of a flexible material and when installed as intended is movable at least between an inoperative position (10) and a sealing position. The servicing section is connected with the gas section to be gas-permeable in the inoperative position. The servicing unit comprises an attachment frame (13) and the sealing device is attached to the attachment frame. The attachment frame is attached to a carrying console (14) of the servicing unit. The sealing device comprises bellows. At least one displaceable guide member such as a guide rope or a guide chain is provided by means of which the sealing device is displaceable, where the retention wall of the sealing device can be descended by means of the guide member at least from the inoperative position to the sealing position. An INDEPENDENT CLAIM is included for a biogas plant comprising: at least one fermenter tank and a fermentation section provided in an interior space of the fermenter tank and a gas section which is closed gas-tight by at least one gas closing wall disposed at the fermenter tank, where at least one agitator device and at least one servicing device with a servicing section are provided, where the agitator device can be transferred at least from an operating position in the fermentation section to a maintenance position in the servicing section of the servicing device, at least one movable sealing device is provided having at least one retention wall of a flexible material and is displaceable between an inoperative position and a sealing position, where in the sealing position the sealing device separates the servicing section from the gas section gas-tight.    DESCRIPTION OF DRAWING(S) - The figure shows enlarged illustration of servicing device of the biogas plant.    Servicing device (2)    Servicing unit, servicing shaft (3)    Fermentation section (5)    Gas section (6)    Sealing device (9)    Inoperative position (10)    Servicing section (12)    Attachment frame (13)    Carrying console (14)    Agitator device (23)    Operating position (24)    Holding rod (26)    Bottom (27)    Tank wall (29)    Agitator blade (31)    Platform (34)