The electrochemical decomposition of CO2 gas in LiCl-Li2O or CaCl2-CaO molten salt was studied to produce carbon. This process consists of the electrochemical reduction of the oxide, Li2O or CaO, and the thermal reduction of CO2 gas by metallic Li or Ca. Two kinds of ZrO2 solid electrolytes were tested as an oxygen ions conductor and removed oxygen ions from the molten salts to the outside of reactor. After the electrolysis in the both salts, the aggregations of nanometer-scale amorphous carbon and rodlike graphite crystals were observed by transmission electron microscope. When 9.7% CO2 Ar mixed gas was blown into LiCl-Li2O or CaCl2-CaO molten salt, the current efficiency was evaluated to be 89.7% or 78.5%, respectively, by the exhaust gas analysis and the supplied charge. When the solid electrolyte with the better ionic conductivity was used, the current and the carbon production became larger. The rate determining step of this proposal was diffusion of oxygen ions in ZrO2 solid electrolyte. 2013 Elsevier Ltd. All rights reserved.