[en] It is important for neutron irradiation tests of materials and fuels to clarify the irradiation environment. Especially, the oxygen and hydrogen peroxide concentrations are required to measure for the analysis of corrosion mechanism of the structure materials in the Light-Water Reactor (LWR) conditions. On the other hand, it is necessary to develop the small and high-accuracy sensor because the outer diameter of the irradiation capsules used in JMTR is limited with φ65mm at a maximum. Thus, the small ceramic gas sensor combined stabilized-zirconia (YSZ) as oxygen ion conductor and platinum as electrode has been developed. In this report, the trial fabrication tests of the ceramic gas sensor carried out by the Spark Plasma Sintering (SPS) method and basic performance results of the sensor were described. (author)