A combustible gas sensor using semiconducting gamma hematitegammaFe2<03 ceramics has been developed. This sensor is made by applying a remarkable gas sensitive phenomenon observed ingammaFe2O3ceramics. Clarification of the gas sensitive behavior and degradation mechanism permitted device life time to be estimated from accelerated testing. This sensor is applicable to liquefied petroleum (LP) gas detectors and gas security systems for which its large sensitivity and high reliability are particularly advantageous. Typical gas sensitive properties, mechanism of operation, and reliability of the sensor are discussed.