NOVELTY - The parallel adsorption towers (11,21,31) comprises sulfur-type and organic-silicon type impurity adsorption agents (12,13), moisture-content adsorption agent (24) and carbon-dioxide adsorption agent (35), respectively arranged along the circulation direction of original gas (1). An exhaust gas re-supply pipe supplies carbon dioxide gas (1d) back to the towers (11,21) from the tower (31). A control unit controls the opening degree of flow control valves (33,37) in exhaust gas re-supply pipe based on level of carbon dioxide measured using carbon dioxide densitometers.    USE - For purification of biogas, waste-material fermentation methane gas (both claimed) of foodstuff or garbage, used as fuel for incineration system, gas turbine, micro-gas turbine, fuel cell for electric power generation etc.    ADVANTAGE - The corrosion of exhaust-gas path by the impurities is prevented reliably and the purity of the gas is increased. The regeneration of the adsorption agents is also performed reliably, so that the processing efficiency is improved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for manufacturing method of methane gas that involves removing sulfur-type impurity (1a) e.g. hydrogen sulfide, organic-silicon type impurity (1b) e.g. siloxane, moisture-content (1c) and carbon dioxide gas from the original gas using sulfur-type and organic-silicon type impurity adsorption agents, moisture-content adsorption agent and carbon-dioxide adsorption agent, respectively.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the gas purification apparatus. (Drawing includes non-English language text)    Original gas (1)    Sulfur-type impurity (1a)    Organic-silicon type impurity (1b)    Moisture content (1c)    Carbon dioxide gas (1d)    Adsorption towers (11,21,31)    Sulfur-type impurity adsorption agent (12)    Organic-silicon type impurity adsorption agent (13)    Moisture-content adsorption agent (24)    Flow control valves (33,37)    Carbon-dioxide adsorption agent (35)