The present invention provides a gas sensor capable of suppressing a variation of a sensor output by a sensor cell to be small. A sensor element of a gas sensor comprises a first solid electrolyte body and a second solid electrolyte body having oxygen ion conductivity, a measured gas chamber into which the measured gas is introduced, a first reference gas chamber and a second reference gas chamber into which reference gas is introduced, a first pump cell, a second pump cell, a sensor cell, and a heater. A value obtained by dividing a first average cross-sectional area of the first reference gas chamber by the first length is larger than the value obtained by dividing the second average cross-sectional area of the second reference gas chamber by the second length.