NOVELTY - Oxygen molar concentration of argon gas containing oxygen, hydrogen, carbon monoxide and nitrogen as impurities, is adjusted to specified value. Oxygen in argon gas is reacted with carbon monoxide and hydrogen in argon gas to obtain carbon dioxide and water. The water-content ratio in argon gas is reduced, and dry carbon monoxide is added to argon gas. Dry carbon monoxide in argon gas is reacted with oxygen in argon gas to obtain carbon dioxide. After specified time, impurity in argon gas is reduced using adsorption agent. Thus, argon gas is purified.    USE - Purification of argon gas used as furnace atmosphere gas in silicon-single-crystal-drawing furnace, ceramic sintering furnace, vacuum-degassing installation for steel manufacture, silicon plasma melting apparatus for solar cell, and polycrystalline-silicon casting furnace.    ADVANTAGE - The method efficiently provides highly purified argon gas with reduced consumption of energy.    DETAILED DESCRIPTION - When the oxygen molar concentration of argon gas containing oxygen, hydrogen, carbon monoxide and nitrogen as impurities, is 1/2 or less with respect to the total of carbon monoxide molar concentration and hydrogen molar concentration, then oxygen is added and the oxygen concentration of argon gas is adjusted to more than 1/2. Oxygen in the argon gas, is reacted with carbon monoxide and hydrogen in the argon gas to obtain carbon dioxide and water, in presence of a catalyst. The water-content ratio in the argon gas is reduced by dehydration process, and dry carbon monoxide is added to the argon gas, such that carbon dioxide molar concentration in the argon gas is 2 times with respect to oxygen molar concentration. Dry carbon monoxide in the argon gas is reacted with oxygen in the argon gas in presence of catalyst to obtain carbon dioxide. After specified time, the content rate of impurity in argon gas is reduced using an adsorption agent. Thus, argon gas is purified. An INDEPENDENT CLAIM is included for purifier (A) of argon gas, which has reactor (1) into which argon gas is introduced, concentration adjusting device (4), drying machine (5) which reduces the water-content ratio of argon gas, reactor (6) into which the argon gas having reduced water-content ratio is introduced, concentration adjusting device (7) which sets carbon monoxide mole concentration in argon gas, and adsorption apparatus (9) connected to reactor (6).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of purifier of argon gas.    Reactors (1,6)    Concentration adjusting devices (4,7)    Drying machine (5)    Adsorption apparatus (9)    Purifier (A)