The invention relates to a process for producing biogas from fibrous substrate by anaerobic fermentation. The process is characterised in that a) the fibrous substrate is fed together with process liquid to a fermenter containing anaerobic microorganisms depending on the TSS content ascertained in this fermenter, b) the fibrous substrate is subjected to wet fermentation in this fermenter to produce biogas c) the output containing fermented fibrous substrate is drawn off from the fermenter and the TSS content in the fermenter is ascertained, d) the TSS content ascertained is compared with a fixed target range and e) depending on the result from d), step a) is repeated with adjusted amounts in order to comply with the target range for the TSS in the fermenter.