NOVELTY - Purifying methane- and hydrogen sulfide-containing crude gases, preferably biogas, comprises (i) desulfurizing the crude gas, (ii) separating carbon dioxide by pressure washing using a washing solution, (iii) dissolving, (iv) removing purified biogas, (v) purifying the loaded washing solution, (vi) supplying the washing solution to pressure washing, (vii) treating the loaded washing solution in at least one depressurization step, (viii) removing nitrogen-, oxygen-, carbon dioxide- and methane-containing gas mixture in a washing step, and (ix) purifying the purified wash solution.    USE - The method is useful for purifying methane- and hydrogen sulfide-containing crude gases, preferably biogas.    ADVANTAGE - The method reduces the methane loss by methane slip, and exhibits improved separation of methane from wash solution.    DETAILED DESCRIPTION - Purifying methane- and hydrogen sulfide-containing crude gases, preferably biogas, comprises (i) desulfurizing the crude gas, (ii) separating carbon dioxide by pressure washing using a physically acting washing solution, which is supplied countercurrently, (iii) dissolving in the washing solution, (iv) removing purified biogas, (v) purifying the loaded washing solution in a first stripping step and a second stripping step, (vi) supplying the washing solution to pressure washing, (vii) treating the loaded washing solution resulting from the pressure washing in at least one depressurization step before the first stripping step, (viii) removing nitrogen-, oxygen-, carbon dioxide- and methane-containing gas mixture discharged from the depressurization step, in a washing step, and (ix) purifying the purified wash solution, which is supplied countercurrently, where high purity methane is produced. An INDEPENDENT CLAIM is also included for a plant for performing the above method, comprising at least one pressure washing column (K1), an expansion column, a first stripping column (S1), a second stripping column and an intermediate washing column, where the washing column is connected with the head of the expansion column via a line (23) to supply nitrogen, oxygen, carbon dioxide and methane-containing gas mixture to the washing column and to supply purified washing water with the sump of the second stripping column via lines.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the plant.    Washing column (K1)    Pump (P2)    Stripping column (S1)    Circulation line (21)    Line (23)