NOVELTY - A mixed gas containing methane and carbon dioxide is contacted with water under normal pressure, and carbon dioxide is dissolved in water. Carbon dioxide is then separated from mixed gas, to obtain a purified gas of highly concentrated methane.    USE - Gas-purification method is used for separating carbon dioxide from mixed gas of methane and carbon dioxide e.g. methane fermentation gas.    ADVANTAGE - The method enables economical production of purified gas of highly concentrated methane (95 vol.% or more). Since cooling of mixed gas and water with a cooler is not required, installation expense and operating cost of gas compressor, feed water pump, or cooler is reduced. Since a high-pressure water-absorption tower is not used, maintenance expense of absorption tower is reduced and direct inlet of mixed gas produced from the fermenter can be carried out to the absorption tower at normal pressure. Hence, the need for installing a buffer tank between fermenter and absorption tower is eliminated. Carbon dioxide is diffused efficiently. The water thermometer and pH meter provide the temperature and pH of water returned to the absorption tower from the stripper. Hence, fresh water can be replenished when the solubility of carbon dioxide in the absorption tower reduces due to increase in water temperature or decrease in pH.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for gas-purification apparatus consisting of a column in which mixed gas is contacted with water, an absorption tower (1) in which carbon dioxide is dissolved in water and separated from mixed gas, and a stripper (2) which introduces carbon-dioxide-dissolved water from the absorption tower and diffuses carbon dioxide under reduced pressure in the column.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view of the gas-purification apparatus. (Drawing includes non-English language text)    Absorption tower (1)    Stripper (2)    Water circulation piping (5)    Water thermometer (7)    Meter for measuring pH (8)    Pressure-reduction diffusion chambers (16,17)