Abstract: The invention relates to a method for producing biogas or sewage gas by a multi-stage anaerobic reaction of biomass and/or sludge. Considering the disadvantages of the known prior art, a method is to be provided that leads to a higher yield of raw gas or biogas and a higher content of methane in the raw gas and enables an economically improved operating method. To this end, the reaction is carried out in the first fermentation stage (F1) while maintaining a TS content of 3 to 8% and a volume load of 1 to 3 kg OTS/m3 d. In the second fermentation stage (F2), a further reaction of the solid matter phase is carried out while maintaining a TS content of 8 to 40% and a volume load of over 2 kg OTS/ m3 d. In the second fermentation stage, the fermentation substrate is set to a TS content that is higher than the TS content of the first stage. In both fermentation stages, the reaction is carried out in the range from slightly acidic to neutral (pH value 6.5 to 8).