A sensor element includes a porous reference gas introduction layer for introducing a reference gas as a reference for detecting a specific gas concentration of a measurement-object gas from an entrance and allowing the reference gas to flow to a reference electrode disposed at the back, wherein the reference gas introduction layer is divided by a partition position defined to be in the range of 50 to 95% of the full length of the reference gas introduction layer from the entrance toward the inside, into a back side portion and an entrance side portion, the reference electrode is accommodated in the back side portion, and the diffusion resistance Rb of the entrance side portion is higher than the diffusion resistance Ra of the back side portion.