NOVELTY - Biogas is introduced into adsorption tower (51,52,53) filled with adsorption agent (51-1,52-1,53-1), carbon dioxide is adsorbed by adsorption agent, methane is extracted, exhaust gas is collected using other adsorption tower, and pressure is equalized using pressure equalization process. Carbon dioxide is purged into the adsorption tower which cleans inside of the adsorption tower and methane is discharged. Carbon dioxide is taken from the adsorption tower as product by evacuating carbon dioxide, and exhaust gas is collected using other adsorption tower.    USE - Manufacture of carbon-dioxide used as sealing gas for welding, and dry-ice raw material for food additives of drinking water.    ADVANTAGE - The method enables manufacture of carbon dioxide having high purify of 99% or more, with high recovery rate of 80% or more, and provides methane having high purity of 90% or more.    DETAILED DESCRIPTION - Biogas is introduced into the adsorption tower filled with adsorption agent, carbon dioxide is adsorbed to the adsorption agent by pressurizing, methane is extracted, exhaust gas is collected using other adsorption tower, and pressure is equalized using pressure equalization process. Carbon dioxide is purged into the adsorption tower which cleans inside of the adsorption tower and methane is discharged. Carbon dioxide is taken out from the adsorption tower as product by evacuating carbon dioxide, and exhaust gas is collected using other adsorption tower. The adsorption process by shifting phase, purge process and exhaust gas collection process are carried out using three adsorption towers. The exhaust gas collection process is terminated when carbon-dioxide adsorption amount of adsorption agent is saturated. The time to implement methane extraction is more than or equal to time to implement carbon dioxide purging process. Carbon dioxide with methane is separated from biogas using pressure-fluctuation adsorption method. An INDEPENDENT CLAIM is included for carbon dioxide manufacturing apparatus (30).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of carbon dioxide manufacturing apparatus. (Drawing includes non-English language text)    Carbon dioxide manufacturing apparatus (30)    Vacuum pump (39)    Tank (45)    Adsorption towers (51,52,53)    Adsorption agents (51-1,52-1,53-1)