NOVELTY - Device comprises a purifying unit for removing siloxane and hydrogen sulfide contained in biogas supplied through a feed line, a pressure swing adsorption unit for removing carbon dioxide from the biogas supplied from the purifying unit, a first return unit (20) for measuring methane concentration of biomethane discharged from pressure swing adsorption unit, a second return unit (30) for measuring methane concentration in residual gas containing carbon dioxide, and (e) a third return unit (40) for measuring methane concentration of purge gas in the pressure swing adsorption unit.    USE - The device is useful for purifying biogas to produce biomethane (claimed).    ADVANTAGE - The device improves recovery of biomethane.    DETAILED DESCRIPTION - Device comprises (a) a purifying unit for removing siloxane and hydrogen sulfide contained in biogas supplied through a feed line, (b) a pressure swing adsorption unit for removing carbon dioxide from the biogas supplied from the purifying unit and for discharging remaining gas containing separated carbon dioxide to outside, (c) a first return unit (20) for measuring methane concentration of biomethane discharged from pressure swing adsorption unit and controlling return of biomethane into the feed line, when concentration of biomethane is less than first reference value, (d) a second return unit (30) for measuring methane concentration in residual gas containing carbon dioxide discharged from pressure swing adsorption unit and controlling return of biomethane into the feed line, when concentration of biomethane is greater than or equal to a second reference value, and (e) a third return unit (40) for measuring methane concentration of purge gas in the pressure swing adsorption unit and controlling return of biomethane into the feed line, when concentration of biomethane is greater than or equal to a third reference value; (a), (b), and (d); (a), (b), and (e). An INDEPENDENT CLAIM is also included for purifying biogas, comprising sequentially removing hydrogen sulfide and siloxane from supplied biogas, separating carbon dioxide from purified biogas via the pressure swing adsorption unit, discharging produced biomethane via first discharge pipe, controlling discharge of residual gas containing carbon dioxide via second discharge pipe, discharging some portion of biogas inside the pressure swing adsorption unit into purge gas via third discharge pipe, and performing at least one of first control by measuring methane concentration of biomethane discharged from pressure swing adsorption unit and controlling return of biomethane into the feed line, when concentration of biomethane is less than first reference value, second control by measuring methane concentration in residual gas containing carbon dioxide discharged from pressure swing adsorption unit and controlling return of biomethane into the feed line, when concentration of biomethane is greater than or equal to a second reference value, and third control by measuring methane concentration of purge gas in the pressure swing adsorption unit and third controlled return of biomethane into the feed line, when concentration of biomethane is greater than or equal to a third reference value.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the device for purifying biogas (Drawing includes non-English language text).    First return unit (20)    First return line (21)    First sensing unit (22)    Second return unit (30)    Third return unit (40)