NOVELTY - Producing biogas by anaerobic fermentation of organic starting materials (2) comprises introducing the organic starting materials in liquid and/or solid form, preferably into a heatable fermentor (1), and subsequently restoring them into a closed secondary fermentor or a final storage for further fermentation, where: the organic starting materials are circulated in a ring channel system (5) of the fermentor comprising an average flow cross-section; and the circulation is brought about by a driving unit (4).    USE - The method is useful for producing biogas by anaerobic fermentation of organic starting materials.    ADVANTAGE - The method: exhibits reduced energy usage in the production of biogas; prevents the formation of sinking- and floating layers effectively; allows a proper degassing; facilitates the introduction of fresh starting materials; and enables a rapid homogenization of the entire substrate volume.    DETAILED DESCRIPTION - Producing biogas by anaerobic fermentation of organic starting materials (2) comprises introducing the organic starting materials in liquid and/or solid form, preferably into a heatable fermentor (1), and subsequently restoring them into a closed secondary fermentor or a final storage for further fermentation, where: the organic starting materials are circulated in a ring channel system (5) of the fermentor comprising an average flow cross-section; and the circulation is brought about by a driving unit (4), which reduces the organic starting materials transported through an adjustable, circulating cross-section compared to the average flow cross-section of the ring channel system, preferably with respect to its vertical position in the fermentor. An INDEPENDENT CLAIM is also included for a gas-tight closed, preferably heatable fermentor for carrying out the above method, comprising a driving unit for circulating the organic starting materials in a ring channel system of the fermentor, where the rotor of the driving unit is surrounded by a flow ring/-tube, which is coupled to an opening of a partition wall (9) filling up a cross-section of the ring channel system, and the opening comprises a circulating cross-section, which is smaller than the average flow cross-section of the ring channel system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the fermentor.    Fermentor (1)    Organic starting material (2)    Driving unit (4)    Ring channel system (5)    Partition wall (9)