PURPOSE: To prevent heat reserve of a catalyst and methanize advantageously carbon oxide, by causing methanation reaction mainly in a riser reaction tube, cooling the catalyst emerging from the above-mentioned reaction tube, and refeeding the cooled catalyst to the inlet of the reaction tube.
CONSTITUTION: A raw material gas containing carbon oxide and hydrogen, together with a methanation catalyst, is fed to an inlet at the lower end of an almost vertical reaction tube 2, and the carbon oxide is methanized while being raised in the reaction tube 2 to produce a gas rich in methane. The resultant gas, together with the above-mentioned catalyst, is then transferred to a collection chamber 3 having the upper end outlet of the reaction tube 2 assembled therein to form a fluidized bed 4 of the above-mentioned catalyst in the bottom region of the collection chamber 3, release the resultant produced gas to the upper region of the collection chamber 3 and recover the gas through a separator 5. The above-mentioned catalyst is then cooled with a cooling tube 6 in the fluidized bed 4 or a separate heat exchanger 7 or a fluidized bed 8 for cooling on the outside of the fluidized bed 4 to remove methanation reaction heat. The catalyst is then circulated through the inlet of the reaction tube 2 to avoid the occurrence of hot spots, and the carbon oxide is advantageously methanized.