A gas sensor element of the present disclosure includes a measurement gas chamber, a solid electrolyte body, and a sensor electrode. The sensor electrode has a noble metal region which contains at least Rh and Pt, an electrolyte region which is formed by a solid electrolyte, and a mixed region in which the noble metal and the solid electrolyte are mixed. With respect to a correlation curve which represents a correlation between a mass percentage concentration c of Rh and a thickness d of the mixed region, when a reaction resistance to a measured gas in the sensor electrode is 40 kΩ, the concentration c of Rh and the thickness d are set so that at coordinates (c, d), the concentration c has a positive coordinate point and the thickness d has a positive coordinate point.