PURPOSE: To continuously and efficiently obtain high-purity carbon dioxide by supplying a specified gas to a combustion chamber and completely burning methane in an oxyhydrogen flame while keeping the pressure in absorption at reduced pressure.

CONSTITUTION: An absorbent of KOH, etc., is injected into a first absorption tower 14 and a second absorption tower 21 from the inlets 10 and 20, a vacuum pump 24 is operated, oxygen 2 and hydrogen 3 are supplied, and a burner 4 is ignited. A three-way cock 16 is adjusted so that the gas is supplied to 15-17 but not to 15-18. At this time, the exhaust combustion gas is allowed to flow through 4-9 to 14-17 to 21-25. After the combustion is stabilized, a gaseous mixture contg. 5-15wt.% CH4, 20-30wt.% H2 and 55-75wt.% O2 is supplied from a feed line 1 to control the towers 14 and 21 to a desired pressure, and the gaseous mixture is burned in a combustion chamber 4. A valve 9 then closed, regeneration is started, the cock 16 is opened to inject an acid into the tower 14 to generate, CO2, which is passed by its pressure through 14-18 to 26-29, and CO2 is recovered from the outlet 29.