To obtain H2 required to fix CO2 at a low cost, convert the CO2 into dimethyl ether using the H2 and fix the CO2.

A produced gas containing CH4 or CO2 is produced by anaerobic fermentation, etc., of an organic waste in an apparatus 1 for treating the organic waste and the resultant produced gas is then fed to a part 2 for separating and enriching methane. The CH4 is purified from the produced gas by passing the produced gas through the part 2 for separating and enriching the methane and then decomposed into carbon (C) and H2 in the presence of a catalyst in a part 3 for decomposing the methane. The H2 produced in the part 3 for decomposing the methane is then fed to an apparatus 4 for producing dimethyl ether to react the CO2 with H2 in the presence of a catalyst to synthesize the dimethyl ether (CH3 COCH3), which is further passed through a condensor 5 to thereby condense the resultant gaseous dimethyl ether. The condensed dimethyl ether is then separated from steam (H2O). Thereby, the liquid dimethyl ether is produced.