To provide a carbon dioxide immobilizing device having a simple structure for high efficiently and inexpensively immobilizing CO2.

CO2 is immobilized by generating a generated gas containing CH4 and CO2 by the anaerobic fermentation of an organic waste in an organic waste treating part 1, sending the generated gas to a reaction vessel 4 serving both as a methane decomposition.hydrogen generating part and a carbon dioxide immobilizing part to produce carbon and water in the presence of a catalyst. As a result, the large consumption of power to obtain H2 is prevented to reduce the cost and the structure is simplified. At the same time, the reaction efficiency is increased and CO2 is never discharged to the outside by sending the unreacted gas in the reaction vessel 4 to a gas mixing.condenser to circulate. Further, excess hydrogen produced in the reaction vessel 4 is separated by a hydrogen separator 8 to utilize again to generation of heat or power to reduce energy supplied from the outside.