For gas sensors, not only sensitivity and selectivity but also fast response speed is very important. We fabricated a restive type gas sensor using ceria (cerium oxide) porous thick film with a particle size of about 100 nm in order to improve the response speed of the sensor. First, we investigated the sensing properties for the change of oxygen partial pressure. The sensor was able to detect the variation of oxygen partial pressure in the wide range from 10-18 Pa to 105 Pa at 873 K. The response time (t90), which is defined as the time until response changes 90% of the saturated value after a sudden change of oxygen partial pressure, was about 10 ms at 1073 K. Next, we investigated the sensing properties for carbon monoxide. We developed the new sensor having two ceria thick films with and without catalyst layer to give selectivity. The sensor was able to detect carbon monoxide over 330 ppm at 773 K, and t90 was 2 s at 723 K. By using Pt/Al2O3 catalyst, the sensor hardly responded for hydrogen, so it was implied that the sensor had a good selectivity for carbon monoxide.