The present invention provides: a method and apparatus for producing a methane-rich gas that has a low carbon monoxide concentration; and a method for decreasing carbon monoxide in a gas that contains carbon monoxide and methane. A method for producing a methane-rich gas, said method comprising: a process wherein hydrogen is added to a gas that contains carbon dioxide so as to cause a CO2 methanation reaction, thereby obtaining a gas that mainly contains methane, while secondarily containing carbon monoxide; and a process wherein oxygen is added to the thus-obtained gas and an oxidation reaction of carbon monoxide is caused in the presence of a CO selective oxidation catalyst. A method for decreasing carbon monoxide in a gas that mainly contains methane, while secondarily containing carbon monoxide, said method comprising a process wherein oxygen is added to a gas that mainly contains methane, while secondarily containing carbon monoxide, and an oxidation reaction of carbon monoxide is caused in the presence of a CO selective oxidation catalyst.
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