We have directly investigated the chemical state of the Pd species in a realμ-gas sensor device by examining theμ-fluorescence X-ray absorption fine structure. Theμ-gas sensor device was heavily damaged by a heating process in which the temperature was ill-controlled, resulting in decrease of methane selectivity. We found that the PdO in the freshμ-gas sensor was reduced to Pd metal particles as the methane selectivity decreased. Based on the investigation results, we modified the device structure so as to heat up homogeneously. The lifetime of the sensor was then successfully increased by more than 5 years.