After complete decomposition of CO2 gas, carbon and oxygen can be used in the steel plant. In addition, CO gas may be used as an effective fuel with easy handling. The preferential decomposition to CO gas is examined by using a combination of CaCl2-CaO melt as a media and ZrO2-8mol% Y2O3 solid-state electrolyte as anode. At a higher electrolysis temperature, CO2 gas is preferentially converted to CO gas. The conversion ratio approached the maximum value of 88.2% at 1273 K. The highest CO concentration was generated at cell voltage of 2.55 V, which is slightly lower than the theoretical decomposition voltage of CaO. Therefore, CO2 decomposes preferentially via the decomposition reaction of carbonate ions. Namely CO2 gas dissolves to form CO32- with coexistence of CaO in the melt, CO32- precipitates carbon or CO gas bubbles on the cathode, and a part of carbon reacts with the blown CO2 gas to form CO gas.