NOVELTY - Biogas is pressure supplied to a primary separation membrane (1), permeate gas containing carbon dioxide which is passed through the primary separation membrane is compressed, and the carbon dioxide contained in the permeate gas is liquefied, non-permeate gas containing methane is supplied to a secondary separation membrane (2), and the permeate gas which is passed through the secondary separation membrane is recirculated to the primary separation membrane. The non-permeate gas which does not pass through the secondary separation membrane is collected as methane enrichment gas.    USE - Method for concentrating methane used for biogas system.    ADVANTAGE - The methane is effectively and efficiently concentrated with suppressed decline in separation efficiency of separation membranes, and moisture content in the permeate gas.    DETAILED DESCRIPTION - Biogas is pressure supplied to a primary separation membrane, permeate gas containing more amount of carbon dioxide which is passed through the primary separation membrane is compressed, and the carbon dioxide contained in the permeate gas is liquefied by coiling, and collected. The permeate gas which is remained is again recirculated to the primary separation membrane. Non-permeate gas containing more amount of methane which does not pass through the primary separation membrane is supplied to a secondary separation membrane by a residual pressure, and the permeate gas which is passed through the secondary separation membrane is recirculated to the primary separation membrane. The non-permeate gas which does not pass through the secondary separation membrane is collected as methane enrichment gas. An INDEPENDENT CLAIM is included for apparatus for concentrating methane, which has a supply pipe (4) with which biogas is supplied, the primary separation membrane module connected to the supply pipe, a primary compressor (5) provided in the supply pipe, a primary recirculation pipe (7) which recirculates the permeate gas, non-permeate gas pipe in which the non-permeate gas is flows, a secondary compressor and cooler respectively provided in the primary recirculation pipe from the upstream, a secondary separation membrane module connected to the non-permeate gas pipe, a secondary recirculation pipe which recirculates the permeate gas, a collection pipe which collects the non-permeate gas as methane enrichment gas.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of apparatus for concentrating methane. (Drawing includes non-English language text)    Primary separation membrane (1)    Secondary separation membrane (2)    Supply pipe (4)    Primary compressor (5)    Primary recirculation pipe (7)