NOVELTY - Digestion tank for producing biogas, comprises (a) a container pot (1) for receiving biomass and a covering made of a film cap (31), (b) a supporting structure (22) that extends in the container cup above the maximum filling level of the biomass, (c) film cap, which spans the container pot over the supporting structure as an expanded gas storage space in a gastight manner, and has a funnel-shaped necking trough that is pulled down to the supporting structure and is maintained gastight through clamping means, (d) an agitator (23), and (e) a drive (34) for the shaft of the agitator.    USE - The digestion tank is useful for producing biogas.    ADVANTAGE - The digestion tank: has simple supporting structure; exhibits optimal stirring action on the container content using the agitator; is easily accessible from outside; and is easy to repair.    DETAILED DESCRIPTION - Digestion tank for producing biogas, comprises (a) a container pot (1) for receiving biomass and a covering made of a film cap (31), (b) a supporting structure (22) that extends in the container cup above the maximum filling level of the biomass, (c) film cap, which spans the container pot over the supporting structure as an expanded gas storage space in a gastight manner, and has a funnel-shaped necking trough that is pulled down to the supporting structure and is maintained gastight through clamping means, (d) an agitator (23) that is maintained at the supporting structure and is placed vertically in the container cup such that the agitator in the region of the funnel-shaped necking trough is guided via a shaft through the supporting structure to the outer side of the film cap, and (e) a drive (34) for the shaft of the agitator, which is arranged in the region of the funnel-shaped necking trough on the outer side of the film cap.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic sectional view of the digestion tank.    Container pot (1)    Supporting structure (22)    Agitator (23)    Film cap (31)    Drive (34)