NOVELTY - Pressurization supply of the biogas is carried out to the first stage separation membrane (1), permeation gas containing carbon dioxides is compressed, liquefied and collected. Remaining of permeation gas is supplied and impervious gas-containing methane, is supplied to the second stage separation membrane (2) by residual pressure. Permeation gas containing carbon dioxides is recirculated to the first stage separation membrane and impervious gas-containing methane is collected as methane enrichment gas. Thus, methane concentration method of biogas is carried out.    USE - Methane concentration method of biogas used as fuel for natural gas vehicle.    ADVANTAGE - The method efficiently enables methane concentration of biogas.    DETAILED DESCRIPTION - Pressurization supply of the biogas is carried out to the first stage separation membrane, permeation gas containing carbon dioxides which permeates the separation membrane is compressed, the carbon dioxides in the permeation gas are liquefied by cooling and collected. The remaining of permeation gas is supplied and utilized for a gas utilization apparatus (26). An impervious gas-containing methane which does not permeate the first separation membrane, is supplied to the second stage separation membrane by residual pressure, permeation gas containing carbon dioxides which permeates the second stage separation membrane is recirculated to the first stage separation membrane and impervious gas- containing methane which does not permeate the second stage separation membrane, is collected as methane enrichment gas. Thus, the methane concentration method of biogas is carried out. An INDEPENDENT CLAIM is included for apparatus for performing methane concentration method of biogas.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram apparatus for performing methane concentration method of biogas. (Drawing includes non-English language text)    First stage separation membrane (1)    Second stage separation membrane (2)    Cooler (10)    Recirculation pipe (11)    Gas utilization apparatus (26)