NOVELTY - Producing biogas or sewage gas by a multistage anaerobic conversion of biomass and/or sewage sludge using wet fermentation, comprises at least two separate fermentation steps, where after the first fermentation step, the digestate is separated into a solid phase and a liquid phase, and the solid phase is further treated and subsequently subjected to a further fermentation process in the second fermentation step. The conversion in the first fermentation step is carried out while maintaining a dry mass content of 3-8% and a volume load of 1-3 kg volume load/m3 d.    USE - The biogas is useful: in cogeneration plants for heating purpose; as energy carriers for supplying into natural gas networks.    ADVANTAGE - The method: produces the crude biogas with high yield and with high methane content, preferably upto 65% and thus the method is economical.    DETAILED DESCRIPTION - Producing biogas or sewage gas by a multistage anaerobic conversion of biomass and/or sewage sludge using wet fermentation, comprises at least two separate fermentation steps, where after the first fermentation step, the digestate is separated into a solid phase and a liquid phase, and the solid phase is further treated and subsequently subjected to a further fermentation process in the second fermentation step. The conversion in the first fermentation step is carried out while maintaining a dry mass content of 3-8% and a volume load of 1-3 kg volume load/m3 d and in the second fermentation step a further conversion of the solid phase is carried out, while maintaining a dry mass content of 8-40% and volume load of greater then 2 kg volume load/m3d, where the fermentation substrate in the second fermentation step is adjusted to a dry mass content, which is greater than the dry mass content of the first step, and the conversion is carried out in two fermentation steps in weakly acidic to neutral range (pH of 6.5-8). The biogas obtained from both the fermentation steps are conducted together or separately conducted and subjected to further purification.