PURPOSE: To contrive the speed-up of the axial flow in the main closed path by a method wherein the sub closed path is provided besides the main closed path for the mixture gas containing CO2 under reflux at a high speed, and a catalyst bath is provided in this sub closed path.

CONSTITUTION: The CO2 gas under reflux to the arrow direction by the discharge between electrode 1, 2 and 3, 4 is put under reflux to oscillation parts 6 via the main closed path including a fan 5. The sub closed path is provided besides this main closed path, and the catalyst bath 7 is provided in this sub closed path. This contitution enables to increase the speed of reflux of the CO2 gas, compared with conventional one provided with the catalyst bath in the main path. CO oxidation can be performed also at the time of oscillation stop by providing a fan 9 in this sub closed path.