CO2 sensors capable of operating at room temperature were fabricated using LaOCl/SnO2 thin films. The carrier mobility of the SnO2 layer was increased by H2 ambient annealing, during which the O vacancies in SnO2 were passivated by H atoms. The sensitivity of the CO2 sensors was strongly affected by the mobility of the SnO2 layer because of the large diffusion length of carriers. By using the SnO2 layer with a carrier mobility of 10.6 cm2 V−1 s−1, a highly sensitive CO2 sensor with a detection limit of 1000 ppm was obtained. The sensor was visible-light-transparent and the transmittance in the visible region was over 70%.