NOVELTY - Biogas is supplied into adsorption tower, and pressure of bio gas is raised. The acid-treated silica gel as water adsorption agent and siloxane adsorption agent, silicalite and acid-treated silicalite as hydrogen sulfide and methyl-mercaptan adsorption agent (5), and lithium, sodium and calcium-A type zeolite which is intercrystallized with aluminum oxide and subjected to silica coating on crystal surface, or lithium, sodium, magnesium and calcium-X type zeolite as carbon dioxide adsorption agent are used in the adsorption tower for purifying biogas.    USE - Purification of biogas such as bio-fermentation gas used for recovering methane used as fuel.    ADVANTAGE - The biogas is purified efficiently using adsorption agent, and methane is effectively recovered.    DETAILED DESCRIPTION - Biogas is supplied into adsorption tower, and pressure of bio gas is raised. The acid-treated silica gel as water adsorption agent and siloxane adsorption agent, silicalite and acid-treated silicalite as hydrogen sulfide and methyl-mercaptan adsorption agent, and lithium, sodium and calcium-A type zeolite which is intercrystallized with aluminum oxide and subjected to silica coating on crystal surface, or lithium, sodium, magnesium and calcium-X type zeolite as carbon dioxide adsorption agent are used in the adsorption tower for purifying biogas. The ratio of silicon oxide and aluminum oxide is 2, and the content of aluminum oxide intercrystallized is 0-5 weight%. Siloxane, hydrogen sulfide, methyl mercaptan and water (sic) are adsorbed under relatively high pressure condition, and methane is separated after adsorbing carbon dioxide. The adsorption agent which adsorbs water, siloxane, hydrogen sulfide, methyl mercaptan and carbon dioxide is provided to relative low pressure condition, and separated. The methane gas and coexisting gas of water, carbon dioxide, siloxane, hydrogen sulfide and methyl mercaptan are recovered by purifying methane gas by pressure swinging method. An INDEPENDENT CLAIM is included for separation of methane and coexistence gas, which involves adsorbing water, siloxane, hydrogen sulfide, methyl mercaptan and carbon dioxide in atmospheric pressure vicinity, and removing under vacuum pressure reduction condition.    DESCRIPTION OF DRAWING(S) - The drawing shows the flow sheet illustrating purification of biogas.    Blower (2)    Packed columns (4a,4b)    Adsorption agent (5)    Valves (8a,8b)    Product tank (12)