NOVELTY - Anaerobic fermentation of organic material involves mixing the organic material to be fermented and an inoculant and supplying the mixture into a fermentation tank (1); moving the mixture from an inlet (6) of the tank to an outlet (9); removing a fraction of the partly fermented material, which is situated between the inlet and the outlet, as an inoculant from the tank via at least one return opening (12); and subjecting the partly fermented material between the return opening and the outlet to an after-fermentation for a certain while before it is removed from the tank via the outlet.    USE - For anaerobic fermentation of biodegradable organic material.    ADVANTAGE - The method of anaerobic fermentation of organic material can be performed in a simple manner. The piece of fresh organic material stay longer in the fermentation tank as the pass-through time is extended. The energetic recycling is maximized as the further produced biogas is collected. The germs or weeds are killed more efficiently. As the partly fermented material is further fermented between the return opening and the outlet for an additional length of time within one and the same reactor, it is not necessary to provide an additional fermentation tank with accessories, which represents a serious economy.    DETAILED DESCRIPTION - Anaerobic fermentation of organic material involves mixing the organic material to be fermented and an inoculant and supplying the mixture into a fermentation tank (1); moving the mixture from an inlet (6) of the fermentation tank to an outlet (9); removing a fraction of the partly fermented material, which is situated between the inlet and the outlet, as an inoculant from the fermentation tank via at least one return opening (12); and subjecting the partly fermented material between the return opening and the outlet to an after-fermentation for a certain while before it is removed from the fermentation tank via the outlet. An INDEPENDENT CLAIM is included for a device for the anaerobic fermentation, comprising a fermentation tank with a fermentation room in which organic material can be fermented; a feed device that can mix fresh organic material with inoculant and can carry it through to an inlet in the fermentation tank, which is also provided with an outlet via which fermented material can be discharged and an outlet for biogas; and a return opening via which a fraction of the partly fermented material, situated between the inlet and the outlet, can be removed from the fermentation tank and can be carried up to the feed device.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a device for the anaerobic fermentation.    Fermentation tank (1)    Mixing pump (4)    Inlet (6)    Outlet (9)    Return opening. (12)