To research the CO2 sensibility of ZnO based on the oxide semiconductor, ZnO, films were prepared with various annealing temperatures and the current–voltage characteristics of the ZnO and SnO2 films were measured in different CO2 ambient conditions using the structure of a metal/semiconductor/Si-wafer. The oxygen vacancy of ZnO changed with the annealing temperatures, and ZnO with the lowest content of oxygen vacancy had two junctions due to its structural matching and degradation of oxygen vacancy at the interface. The oxide semiconductor with two junctions displayed good sensibility of CO2 gas. The effect of the two junctions at the ZnO and SnO2 thin films were observed by the annealing treatment at 110 °C. The oxygen vacancy was closely related with the conductivity of the oxide semiconductor and sensibility of CO2 gas. The two-junction effect for the Schottky contact to assure a high quality semiconductor device will contribute to further strategic researches to develop the ionic gas sensor application.