An anaerobic fermentation part generates gas containing methane and carbon dioxide by anaerobically fermenting biomass. The gas is passed to a methane.carbon dioxide separation part to be separated into methane and carbon dioxide, and part of the separated methane is fed to a methane reforming part to be decomposed into carbon and hydrogen under the presence of a catalyst. The carbon dioxide separated in the methane.carbon dioxide separation part and the hydrogen formed in the methane reforming part are fed to a carbon dioxide fixing part, which in turn reacts the carbon dioxide with the hydrogen under the presence of a catalyst for continuously forming carbon and hydrogen and fixing the carbon dioxide.