NOVELTY - The method comprises anaerobically fermenting a biomass by a liquid fermentation in a fermenter (11) as a fermentation mass (14), and mixing the fermentation mass in the fermenter during the anaerobic fermentation process for producing biogas from the fermentation mass. A first injection device (18) has a vertical circulation and performs through mixing of the fermentation mass. The biogas is injected through an opening on an upper side of pipelines and/or tubes (21). The pipelines and/or tubes comprise further holes on their underside.    USE - The method is useful for producing biogas (claimed).    ADVANTAGE - The method is capable of reliably and effectively mixing the fermentation mass, thus reducing fault-related failures in the production of biogas.    DETAILED DESCRIPTION - The method comprises anaerobically fermenting a biomass by a liquid fermentation in a fermenter (11) as a fermentation mass (14), and mixing the fermentation mass in the fermenter during the anaerobic fermentation process for producing biogas from the fermentation mass. A first injection device (18) has a vertical circulation and performs through mixing of the fermentation mass. The biogas is injected through an opening on an upper side of pipelines and/or tubes (21). The pipelines and/or tubes comprise further holes on their underside, where the further holes clean the opening and the pipelines and/or tubes once per day and/or for a period of 10 minutes. The biogas is injected with a high gas pressure and/or with an increased amount of gas in the pipes and/or tubes in comparison with the biogas for mixing the fermentation mass. A second injecting device has a horizontal circulation and/or carried out the mixing of the fermentation mass. The biomass is mixed with respect to the horizontal circulation of the second injection device in the fermenter. A part of the fermentation mass in the fermenter is passed into a clarifier for the discharge of purified water and is recycled in the clarifier. A part of the fermentation mass of the fermenter is introduced into an aerobic treatment zone for supplying anaerobic bacteria with nutrients, and is returned to the fermenter after flowing through the aerobic treatment zone. An INDEPENDENT CLAIM is included for an apparatus for producing biogas from biomass.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic side sectional view of a device for producing biogas.    Fermenter (11)    Lid (12)    Fermentation mass (14)    First injection device (18)    Pipelines and/or tubes. (21)