TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The first return unit has a first return line (21) connected with a first discharge pipe for discharging the produced biomethane at one end and to the feed line at other end, a first sensing unit (22) to measure methane concentration of biomethane discharged into the first discharge pipe, and a first switching valve for controlling opening and closing to optionally return biomethane into the feed line according to measuring value of the first sensing unit, installed on a coupled part of the first return line and the first discharge pipe. The second return unit has a second return line connected to a second discharge pipe to discharge residual gas containing carbon dioxide from the pressure swing adsorption unit at one end and to feed line at other end, a second sensing unit for measuring methane concentration in the residual gas, connected to the second discharge pipe, and a second switching valve for controlling opening and closing to optionally return residual gas into the feed line according to measuring value of the second sensing unit, installed on a coupled part of the second return line and the second discharge pipe. The third return unit has a third return line connected to a third discharge pipe to discharge purge gas of the pressure swing adsorption unit at one end and to feed line at other end, a third sensing unit for measuring methane concentration in the purge gas, connected to the third discharge pipe, and a third switching valve for controlling opening and closing to optionally return purge gas into the feed line according to measuring value of the third sensing unit, installed on a coupled part of the third return line and the third discharge pipe. The pressure swing adsorption unit is controlled to return biomethane produced for a period of time into the feed line via the first return line. The purifying unit has a desulfurization tower to remove hydrogen sulfide from supplied biogas, and a siloxane removing tower to remove siloxane from hydrogen sulfide removed biogas. The first control comprises measuring methane concentration of biomethane discharged into the first discharge pipe via the first sensing unit, and controlling the first switching valve to return biomethane via the first return line connected to the feed line, when concentration of biomethane is less than first reference value. The second control comprises measuring methane concentration in residual gas containing carbon dioxide discharged into the second discharge pipe via the second sensing unit, and controlling the second switching valve to return biomethane via the second return line connected to the feed line, when concentration of biomethane is greater than or equal to a second reference value. The third control comprises measuring methane concentration of purge gas into the third discharge pipe via the third sensing unit, and controlling the third switching valve to return biomethane via the third return line connected to the feed line, when concentration of biomethane is greater than or equal to a third reference value. Preferred Method: The method comprises controlling the first switching valve to return biomethane into the feed line via the first return line.