A gas sensor element includes a solid electrolyte body, a measurement electrode film, and a measured-gas intake port. The measurement electrode film includes a noble metal region, a solid electrolyte region, and a mixed region in which a noble metal and a solid electrolyte are mixed together. The measurement electrode film has a structure in which a first average thickness of the mixed region within a first region is greater than a second average thickness of the mixed region within a second region that is closer to a center of the measurement electrode film than the first region in a direction along an intake direction of a measured gas is. The first region is a region from an electrode end on the measured-gas intake port side to ¼ of an overall length of the measurement electrode film in the direction along the intake direction of the measured gas.